Bump com.google.protobuf:protobuf-gradle-plugin from 0.8.18 to 0.9.4

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]>

build.gradle

@@ -8,7 +8,7 @@ buildscript {
         mavenCentral()
     }
     dependencies {
-        classpath 'com.google.protobuf:protobuf-gradle-plugin:0.8.18'
+        classpath 'com.google.protobuf:protobuf-gradle-plugin:0.9.4'
     }
 }