Check and adopt Gradle "isolated projects" feature

[StefMa]

Summary
Evaluate adopting Gradle "isolated projects" feature to improve plugin isolation, avoid accidental cross-project state, and improve compatibility with newer Gradle features. See Gradle docs: https://docs.gradle.org/current/userguide/isolated_projects.html

Why

• This plugin uses AGP variant APIs and registers tasks per variant; isolating the plugin can prevent unexpected interactions in multi-module Android projects.
• Improve predictability and compatibility with Gradle configuration cache and classloader isolation.

What to check

• Review apply(project: Project) and variant-specific code for cross-project assumptions.
  • File: src/main/kotlin/com/ioki/sentry/proguard/gradle/plugin/SentryProguardGradlePlugin.kt
• Review extension and CLI config for shared mutable state.
  • File: src/main/kotlin/com/ioki/sentry/proguard/gradle/plugin/SentryProguardExtension.kt
• Validate AGP compatibility and tests (see src/test/kotlin and androidTestProject).

Suggested changes

• Experiment enabling isolated projects and fix issues.
• Add tests for multi-module Android projects and plugin consumption scenarios.
• Document minimum Gradle/AGP versions if required.

Notes

• Test AGP variant API interactions carefully — isolated projects may affect lifecycle or classloading.
• Isolated projects may require bumping minimum Gradle version