Annotate KotlinCompilation.kt with experimental

[wangk5]

Add ExperimentalCompilerApi annotation to KotlinCompilation.kt

[tschuchortdev]

Is this needed? I thought it would be covered by the compiler flags here. If not, then other classes probably need the annotation as well.

[wangk5]

@tschuchortdev Hmm, it looks like AbstractKotlinCompilation is annotated with ExperimentalCompilerApi. Since KotlinCompilation is extending this class, it is returning an experimental compiler api error without adding the annotation

[wangk5]

@tschuchortdev added the annotation to KspTest as well and all checks are now passing!