Bump protobuf-java from 3.15.5 to 3.15.6

Bumps [protobuf-java](https://github.com/protocolbuffers/protobuf) from 3.15.5 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@v3.15.5...v3.15.6)

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

pom.xml

@@ -43,7 +43,7 @@
     <snakeyaml.version>1.28</snakeyaml.version>
     <slf4j.version>1.7.30</slf4j.version>
     <caffeine.version>2.9.0</caffeine.version>
-    <protobuf.version>3.15.5</protobuf.version>
+    <protobuf.version>3.15.6</protobuf.version>
     <junit.version>4.13</junit.version>
     <bucket4j.version>6.0.1</bucket4j.version>
     <bouncycastle.version>1.68</bouncycastle.version>