minimum supported JDK version to run Error Prone is JDK 21 as of Error Prone 2.43.0

### Expected Behavior

use Error Prone 2.43.0.

related might later on use:
- https://docs.openrewrite.org/recipes/java/migrate/upgradetojava21
- https://github.com/checkstyle/checkstyle/blob/0500cab4436f0b2759f5797800ef3e0073454a69/rewrite.yml#L15
- https://github.com/diffplug/spotless/pull/2756

### Current Behavior (optional)

_No response_

### Context

- https://github.com/google/error-prone/issues/5365
- https://github.com/gradle/gradle/issues/35768
- https://github.com/diffplug/spotless/issues/2755

Start by inspecting the repository's current Error Prone configuration and related Gradle build files; the issue names no specific file. Compare the required JDK 21 baseline with Error Prone 2.43.0 and review the linked upstream references. Done means the project uses Error Prone 2.43.0 and its existing checks remain compatible with JDK 21.

Written by the indexing model from the issue text.