Bump compile-testing from 0.19 to 0.21.0

Bumps [compile-testing](https://github.com/google/compile-testing) from 0.19 to 0.21.0.
- [Release notes](https://github.com/google/compile-testing/releases)
- [Commits](google/compile-testing@compile-testing-0.19...v0.21.0)

---
updated-dependencies:
- dependency-name: com.google.testing.compile:compile-testing
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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

dependencies.gradle

@@ -14,7 +14,7 @@ ext {
 
     // DEPENDENCIES
     asciidoctorGradlePluginVersion = "3.3.2"
-    compileTestingVersion          = "0.19"
+    compileTestingVersion          = "0.21.0"
     groovyVersion       = "2.4.21"
     hamcrestCoreVersion = "2.2"
     ivyVersion          = "2.5.1"