build(deps-dev): 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:development
  update-type: version-update:semver-minor
...

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

apt-test-generator/pom.xml

@@ -48,7 +48,7 @@
     <dependency>
       <groupId>com.google.testing.compile</groupId>
       <artifactId>compile-testing</artifactId>
-      <version>0.19</version>
+      <version>0.21.0</version>
       <scope>test</scope>
     </dependency>
     <dependency>