Update MSBuild Server docs: enabled by default in .NET 11 SDK

[Jan Provazník (JanProvaznik)]

Summary

MSBuild Server is enabled by default for MSBuild-based .NET CLI commands (dotnet build, dotnet msbuild) starting with the .NET 11 SDK. This page still described it as opt-in, and its opt-out guidance is now out of date. See dotnet/sdk#55231 (milestone 11.0-preview7), which flips DOTNET_CLI_USE_MSBUILD_SERVER to default true.

Changes

• Default is now on. ## Enable MSBuild Server documents the .NET 11 default and keeps a sentence about the previous opt-in behavior for readers on earlier SDKs.
• Removed the change wave 17.4 opt-out. This was the most important correction. Wave 17.4 has left the current change wave rotation, so SET MSBuildDisableFeaturesFromVersion=17.4 is now treated as out of rotation and enables all features rather than disabling MSBuild Server. Following the old instruction produces the opposite of the intended effect. MSBuild Server is no longer change-wave gated at all.
• Corrected the opt-out value. DOTNET_CLI_USE_MSBUILD_SERVER is documented as false.
• Documented -mt. Multithreaded builds engage MSBuild Server, because multithreaded execution runs project work inside the server process.
• Documented in-process fallback. Invocations that cannot be hosted in a separate process (-help, -version, binary log replay) fall back to building in the launching process.
• Added diagnostics guidance. Builds that request the server record whether one was started, reused, or not used and why; visible at -v:diag or in a binary log.

Verification

Claims were checked against the MSBuild implementation on main:

• Default flip and opt-out precedence: MSBuildForwardingAppWithoutLogging.cs in dotnet/sdk (DOTNET_CLI_USE_MSBUILD_SERVER defaults to true; MSBUILDUSESERVER is only set when the user has not set it).
• 15-minute idle shutdown: CommunicationsUtilities.NodeConnectionTimeout (900 * 1000 ms).
• /nodemode:8: NodeMode.OutOfProcServerNode = 8.
• Change wave rotation: ChangeWaves.AllWaves currently starts at 17.10, so 17.4 is out of rotation.
• -mt implies server: XMake.ShouldUseMSBuildServer.

Notes for reviewers

• ms.date refreshed; author/ms.author updated to the current owner.
• Only the .NET 11 timing statement is forward-looking; everything else describes shipped behavior.

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[learn-build-service-prod]

Learn Build status updates of commit 6914efa:

✅ Validation status: passed

File	Status	Preview URL	Details
docs/msbuild/msbuild-server.md	✅Succeeded

For more details, please refer to the build report.

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PRMerger Results

Issue	Description
File Change Percent	This PR contains file(s) with more than 30% file change.