Bump com.google.errorprone:error_prone_core from 2.19.1 to 2.21.1

Bumps [com.google.errorprone:error_prone_core](https://github.com/google/error-prone) from 2.19.1 to 2.21.1.
- [Release notes](https://github.com/google/error-prone/releases)
- [Commits](google/error-prone@v2.19.1...v2.21.1)

---
updated-dependencies:
- 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>

build.gradle

@@ -97,7 +97,7 @@ dependencies {
     testCompile("org.spockframework:spock-core:2.2-groovy-2.5") {
         exclude module: 'groovy-all'
     }
-    errorprone("com.google.errorprone:error_prone_core:2.19.1")
+    errorprone("com.google.errorprone:error_prone_core:2.21.1")
 }
 
 configurations {