Update to Gradle 9.0.0 and enable the configuration cache

[wmdietl]

There is an issue with the CF Gradle plugin, which causes e.g. the failure in docs/examples/errorprone:

> Task :createCheckerFrameworkManifest
> Task :compileJava FAILED

FAILURE: Build failed with an exception.

* What went wrong:
Execution failed for task ':compileJava'.
> Cannot mutate the dependencies of configuration ':checkerFramework' after the configuration was resolved. After a configuration has been observed, it should not be modified.

* Try:
> Run with --stacktrace option to get the stack trace.
> Run with --info or --debug option to get more log output.
> Run with --scan to generate a Build Scan (Powered by Develocity).
> Get more help at https://help.gradle.org.

BUILD FAILED in 656ms
2 actionable tasks: 2 executed

I don't know whether there is a change in the configuration that could fix this or whether the plugin itself needs to be updated.