Bump com.google.protobuf:protobuf-gradle-plugin from 0.9.2 to 0.9.3 (#141)

Bumps [com.google.protobuf:protobuf-gradle-plugin](https://github.com/google/protobuf-gradle-plugin) from 0.9.2 to 0.9.3.
- [Release notes](https://github.com/google/protobuf-gradle-plugin/releases)
- [Commits](https://github.com/google/protobuf-gradle-plugin/compare/0.9.2...v0.9.3)

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

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

java-spiffe-core/build.gradle

@@ -4,7 +4,7 @@ buildscript {
     }
 
     dependencies {
-        classpath group: 'com.google.protobuf', name: 'protobuf-gradle-plugin', version: '0.9.2'
+        classpath group: 'com.google.protobuf', name: 'protobuf-gradle-plugin', version: '0.9.3'
     }
 }