Bump com.google.protobuf:protobuf-gradle-plugin from 0.8.18 to 0.9.4 (#421)

Bumps [com.google.protobuf:protobuf-gradle-plugin](https://github.com/google/protobuf-gradle-plugin) from 0.8.18 to 0.9.4.
- [Release notes](https://github.com/google/protobuf-gradle-plugin/releases)
- [Commits](google/protobuf-gradle-plugin@v0.8.18...v0.9.4)

---
updated-dependencies:
- dependency-name: com.google.protobuf:protobuf-gradle-plugin
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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

Author: dependabot[bot]

compiler/build.gradle.kts

@@ -25,7 +25,7 @@ dependencies {
     testImplementation("junit:junit:4.13.2")
     testImplementation("com.google.guava:guava:29.0-jre")
     testImplementation("com.google.jimfs:jimfs:1.3.0")
-    testImplementation("com.google.protobuf:protobuf-gradle-plugin:0.8.18")
+    testImplementation("com.google.protobuf:protobuf-gradle-plugin:0.9.4")
     testImplementation("com.nhaarman.mockitokotlin2:mockito-kotlin:2.2.0")
     testImplementation("org.junit.jupiter:junit-jupiter-engine:5.10.0")
     testImplementation("org.mockito:mockito-core:4.11.0")