Improve Gradle 9 compatibility

[jyjeanne]

Description

@infotexture Upgrade build.gradle to be fully compatible with Gradle 9.0 and Gradle's Configuration Cache.

Key changes:

• Replace saxon-gradle plugin with an inline XsltTransformTask class (saxon-gradle is not Configuration Cache compatible)
• Replace deprecated project.exec() with providers.exec() for git commit hash retrieval
• Replace deprecated buildDir with layout.buildDirectory
• Use project.layout.projectDirectory.file() in DSL methods for Configuration Cache compatibility
• Replace internal org.gradle.internal.os.OperatingSystem API with System.getProperty('os.name')
• Add proper error handling, task groups, and descriptions

Motivation and Context

Gradle 9.0 removes several deprecated APIs and makes Configuration Cache the recommended execution mode. The previous build had two incompatibilities:

1. exec deprecation: project.exec() was deprecated in Gradle 8.x and removed in Gradle 9.0
2. Saxon-Gradle incompatibility: The eerohele/saxon-gradle plugin stores non-serializable XmlSlurper objects, breaking Configuration Cache

This change ensures the documentation build continues to work with Gradle 9.0+.

How Has This Been Tested?

Test	Gradle 8.14.3	Gradle 9.0.0
./gradlew help --warning-mode all	PASS	PASS
./gradlew help --warning-mode fail	PASS	PASS
./gradlew --configuration-cache (store)	PASS	PASS
./gradlew --configuration-cache (reuse)	PASS	PASS
./gradlew generatePlatformFilter gitMetadata	PASS	PASS

Note: Full dist build requires DITA-OT installation and was not tested in isolation.

Type of Changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)

Documentation and Compatibility

• What documentation changes are needed for this feature?
  • None required - this is a build infrastructure change only
• Will this change affect backwards compatibility or other users' overrides?
  • No impact on documentation output
  • Maintains backward compatibility with Gradle 8.x
  • Enables forward compatibility with Gradle 9.x
  • The XsltTransformTask DSL is identical to the previous SaxonXsltTask

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[infotexture]

Tested distribution build with feature/gradle9 branch from dita-ot/dita-ot#4740.

✅ Works, no more deprecation messages from Gradle 9. 🎉

Thanks @jyjeanne! 🙏

Merging…