Allow enabling KSP2 per module

[ansman]

With the change in #2034 a side effect was that KSP2 had to be enabled per Gradle project, not per module.

With this change a DSL property has been added to handle this.

[ting-yuan]

The test case needs to be updated:

    * What went wrong:
    Script compilation error:

      Line 7: ksp { useKsp2.set(true) }
                    ^ This API is experimental.It may be changed in the future without notice or might be removed.

Also lint failed in another place:

> Task :gradle-plugin:ktlint FAILED
/home/runner/work/ksp/ksp/gradle-plugin/src/main/kotlin/com/google/devtools/ksp/gradle/KspSubplugin.kt:68:1: Unused import