Bump protobuf.version from 3.15.1 to 3.15.6

Bumps `protobuf.version` from 3.15.1 to 3.15.6.

Updates `protobuf-java-util` from 3.15.1 to 3.15.6
- [Release notes](https://github.com/protocolbuffers/protobuf/releases)
- [Changelog](https://github.com/protocolbuffers/protobuf/blob/master/generate_changelog.py)
- [Commits](protocolbuffers/protobuf@v3.15.1...v3.15.6)

Updates `protobuf-java` from 3.15.1 to 3.15.6
- [Release notes](https://github.com/protocolbuffers/protobuf/releases)
- [Changelog](https://github.com/protocolbuffers/protobuf/blob/master/generate_changelog.py)
- [Commits](protocolbuffers/protobuf@v3.15.1...v3.15.6)

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

pom.xml

@@ -25,7 +25,7 @@
         <sonar.moduleKey>${project.artifactId}</sonar.moduleKey>
         <sonar.projectName>jet-train</sonar.projectName>
         <sonar.sources>src/main/kotlin</sonar.sources>
-        <protobuf.version>3.15.1</protobuf.version>
+        <protobuf.version>3.15.6</protobuf.version>
     </properties>
     <build>
         <sourceDirectory>src/main/kotlin</sourceDirectory>