Make Protobuf Gradle Plugin AGP 9 compatible

[omarismail94]

Establish AGP 8.7 as the minimum supported version and adhere to the new Variant APIs. This ensures forward compatibility with AGP 9.0 while optimizing for the majority of the active ecosystem.

To achieve forward compatibility with AGP 9, the plugin must eliminate all reliance on the "Old Variant API" (BaseVariant, AppExtension, registerJavaGeneratingTask).

The PR does the following

• Target Minimum Gradle: 8.9 (Required by AGP 8.7).
• Target Java Version: Java 17 (Required by AGP 8.0+).
• Target Minimum AGP: 8.7

In the first commit, we upgrade to Gradle 8.9 and Java 17

• Upgrade Gradle Wrapper to 8.9 in project and examples.
• Update build.gradle dependencies and configuration for Gradle 8.9 compatibility.
• Update Spock to version 2.3-groovy-3.0.
• Fix CodeNarc violations and resolve Groovy static compilation issues.
• Update Android test projects to use 'namespace' DSL and remove 'package' attribute from manifests to support Android Gradle Plugin 8.0+.
• Update test matrices to use AGP versions compatible with Gradle 8.9 (e.g., 8.1.0, 8.3.0, 8.13.0).
• Fix 'assertJavaCompileHasProtoGeneratedDir' verification in test projects to work with configuration caching changes.

In the second commit we upgrade to AGP 8.7.3 and migrate to the variant API:

• Upgrade project dependency to Android Gradle Plugin 8.7.3.
• Migrate ProtobufPlugin to use the AndroidComponentsExtension (Variant API) instead of the deprecated BaseVariant API.
• Replace registerJavaGeneratingTask with variant.sources.java.addGeneratedSourceDirectory for source registration.
• Remove ProjectExt and legacy variant iteration logic.
• Remove deprecated AndroidSourceSet extension in Kotlin.

FIXES: Issue #787 , #793

TESTED:

• Compilation passed: Ran ./gradlew assemble
• All tests passed: Ran ./gradlew test
• Published local snapshot, uploaded to AndroidX, and set android.newDsl to true and the build passed. See here

[michnovka]

Hello, wondering if this is merge ready, or we need something to be done? This is the last blocker for us to move to AGP9 fully. Thanks!

[omarismail94]

@michnovka Im just waiting for someone from the owning team to review the change. The regular reviewer is out of office this week, but will try to find someone else. If I don't, then expect it to be reviewed next week