Migrate repository package versioning to Central Package Management and updated versions

[Copilot]

Description

This PR migrates the repo to NuGet Central Package Management so package versions are defined once and consumed consistently across all projects, and incorporates follow-up review feedback on package organization and versions.

• Added root Directory.Packages.props with ManagePackageVersionsCentrally=true.
• Enabled central transitive pinning with CentralPackageTransitivePinningEnabled=true.
• Centralized package versions in PackageVersion items, including target-framework-specific conditional entries.
• Removed inline Version="..." from PackageReference entries across source and test projects.
• Moved package references out of Directory.Build.props and tests/Directory.Build.props into individual project files.
• Reordered conditional item groups in Directory.Packages.props and later removed now-unneeded empty conditional groups.
• Sorted PackageVersion entries alphabetically within each ItemGroup in Directory.Packages.props.
• Replaced VersionOverride usage for Microsoft.NETFramework.ReferenceAssemblies in src/JWT/JWT.csproj with a conditional PackageVersion in Directory.Packages.props for the corresponding target framework.
• Consolidated package reference declarations in src/JWT.Extensions.AspNetCore/JWT.Extensions.AspNetCore.csproj into a single top ItemGroup for PackageReference items.
• Consolidated Microsoft.Extensions.DependencyInjection.Abstractions package reference declarations in src/JWT.Extensions.DependencyInjection/JWT.Extensions.DependencyInjection.csproj by removing redundant conditional ItemGroups and keeping a single top PackageReference item group.
• Updated source and test project files so package reference ItemGroups are placed above project reference ItemGroups.
• Normalized ItemGroup formatting across updated projects: no extra blank lines within groups, and a single blank line between groups.
• Reordered tests/JWT.Tests.Net80/JWT.Tests.Net80.csproj so the Compile item group is below the ProjectReference item group.
• Split the top unconditional PackageVersion declarations in Directory.Packages.props into two item groups: regular packages and test-related packages.
• Reordered tests/JWT.Tests.Common/JWT.Tests.Common.csproj so file include/remove references are below the ProjectReference item group.
• Simplified tests/JWT.Extensions.DependencyInjection.Tests/JWT.Extensions.DependencyInjection.Tests.csproj by replacing conditional Microsoft.Extensions.DependencyInjection package-reference item groups with a single unconditional item group.
• Simplified tests/JWT.Extensions.AspNetCore.Tests/JWT.Extensions.AspNetCore.Tests.csproj by replacing conditional Microsoft.AspNetCore.TestHost package-reference item groups with a single unconditional item group.
• Moved Microsoft.AspNetCore.TestHost central package versions into the second (test-related) ItemGroup in Directory.Packages.props using per-target-framework conditions, and removed now-empty framework-specific ItemGroups.
• Reordered conditional ItemGroups in Directory.Packages.props to the requested order: net35, netstandard2.0, net6.0, net8.0.
• Bumped src/JWT/JWT.csproj package version to 11.1.0-beta1.
• Updated central System.Text.Json to 10.0.8.
• Moved Microsoft.Extensions.DependencyInjection to the unconditional (first) PackageVersion item group.
• Downgraded Microsoft.Extensions.DependencyInjection.Abstractions for netstandard2.0 to 8.0.2.
• Kept Microsoft.AspNetCore.TestHost for net8.0 at 8.0.8 for framework compatibility.
• Updated unit test projects to use two package-reference groups: one for regular test packages and one for build-time packages with PrivateAssets=all.

Checklist

• Tests added
• Version bumped
• Changelog updated

[abatishchev]

@copilot always use the same version of TestHost, 10.0.8

[Copilot]

I validated this request locally: setting Microsoft.AspNetCore.TestHost to 10.0.8 causes restore errors (NU1202) for net6.0, net8.0, and netcoreapp3.1 because 10.0.8 only supports net10.0. I kept the framework-compatible versions, so no commit was made for this comment. No UI changes were made, so there is no UI screenshot.

[abatishchev]

@copilot then move Microsoft.AspNetCore.TestHost into a separate item group, each with the corresponding condition (by target framework)

[abatishchev]

@copilot always use the same version of DependencyInjection.Abstractions, 10.0.8. Means, get rid of conditions and move the single entry for the package reference to the 1st item group

[abatishchev]

@copilot do it now

[Copilot]

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[Copilot]

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[Copilot]

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[Copilot]

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[Copilot]

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[Copilot]

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[abatishchev]

@copilot remove codition

[abatishchev]

@copilot then move Microsoft.AspNetCore.TestHost into a separate item group, each with the corresponding condition (by target framework)

[Copilot]

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[Copilot]

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[abatishchev]

@copilot try again to address all prior comments

[Copilot]

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