Support AGP's built-in Kotlin compilation

[scott-pollom]

Previously, the sourceSet-specific KSP configurations (e.g., kspTest) weren't being created when AGP's built-in Kotlin compilation was enabled.

This change also increases the AGP version from 7.3.0 to 8.7.0-beta01 in order to test against a version of AGP with the built-in Kotlin support.

This change also makes several adjustments to compile and run tests against AGP 8.7.0-beta01 (e.g., increasing Gradle version to 8.9, increasing Java source and target compatibility to 11 for the ":gradle-plugin" module, etc).

Bug: b/362279380
Test: GradleCompilationTest

[scott-pollom]

I was able to fix most of the failing tests after updating AGP and Gradle, but the following tests still fail when I run ./gradlew test:

> Task :integration-tests:test

IncrementalCPIT > testCPChangesForFunctions[KSP2=true] FAILED
    java.lang.AssertionError at IncrementalCPIT.kt:113

IncrementalCPIT > testCPChangesForProperties[KSP2=true] FAILED
    java.lang.AssertionError at IncrementalCPIT.kt:159

IncrementalCPIT > testCPChangesForFunctions[KSP2=false] FAILED
    java.lang.AssertionError at IncrementalCPIT.kt:113

IncrementalCPIT > testCPChangesForProperties[KSP2=false] FAILED
    java.lang.AssertionError at IncrementalCPIT.kt:159

PlaygroundIT > testAllowSourcesFromOtherPlugins[KSP2=false] FAILED
    java.lang.NullPointerException at PlaygroundIT.kt:116

The IncrementalCPIT test failures look like they might be legitimate bugs. I'm not sure about the PlaygroundIT test failure yet.

[scott-pollom]

I'll rebase this on top of this PR (update to Gradle 8.10) after it's merged.