Bump versions.errorprone from 2.20.0 to 2.21.0 (#3076)

Bumps `versions.errorprone` from 2.20.0 to 2.21.0.

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)

Updates `com.google.errorprone:error_prone_test_helpers` 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_core
  dependency-type: direct:production
  update-type: version-update:semver-minor
- dependency-name: com.google.errorprone:error_prone_test_helpers
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

gradle/dependencies.gradle

@@ -6,7 +6,7 @@ def versions = [:]
 
 versions.bytebuddy = '1.14.5'
 versions.junitJupiter = '5.10.0'
-versions.errorprone = '2.20.0'
+versions.errorprone = '2.21.0'
 
 libraries.junit4 = 'junit:junit:4.13.2'
 libraries.junitJupiterApi = "org.junit.jupiter:junit-jupiter-api:${versions.junitJupiter}"