Bump error-prone.version from 2.20.0 to 2.21.0

Bumps `error-prone.version` from 2.20.0 to 2.21.0.

Updates `com.google.errorprone:error_prone_annotations` from 2.20.0 to 2.21.0
- [Release notes](https://github.com/google/error-prone/releases)
- [Commits](google/error-prone@v2.20.0...v2.21.0)

Updates `com.google.errorprone:error_prone_core` from 2.20.0 to 2.21.0
- [Release notes](https://github.com/google/error-prone/releases)
- [Commits](google/error-prone@v2.20.0...v2.21.0)

---
updated-dependencies:
- dependency-name: com.google.errorprone:error_prone_annotations
  dependency-type: direct:production
  update-type: version-update:semver-minor
- dependency-name: com.google.errorprone:error_prone_core
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <support@github.com>

pom.xml

@@ -89,7 +89,7 @@
     <jacoco-maven-plugin.version>0.8.10</jacoco-maven-plugin.version>
     <maven-pitest-plugin.version>1.14.2</maven-pitest-plugin.version>
     <pitest-junit5-plugin.version>1.2.0</pitest-junit5-plugin.version>
-    <error-prone.version>2.20.0</error-prone.version>
+    <error-prone.version>2.21.0</error-prone.version>
     <nullaway.version>0.10.11</nullaway.version>
     <revapi-maven-plugin.version>0.15.0</revapi-maven-plugin.version>
     <revapi-java.version>0.28.1</revapi-java.version>