Bump protobufVersion from 3.15.7 to 3.15.8 (#22)

Bumps `protobufVersion` from 3.15.7 to 3.15.8.

Updates `protobuf-java` from 3.15.7 to 3.15.8
- [Release notes](https://github.com/protocolbuffers/protobuf/releases)
- [Changelog](https://github.com/protocolbuffers/protobuf/blob/master/generate_changelog.py)
- [Commits](protocolbuffers/protobuf@v3.15.7...v3.15.8)

Updates `protobuf-java-util` from 3.15.7 to 3.15.8
- [Release notes](https://github.com/protocolbuffers/protobuf/releases)
- [Changelog](https://github.com/protocolbuffers/protobuf/blob/master/generate_changelog.py)
- [Commits](protocolbuffers/protobuf@v3.15.7...v3.15.8)

Updates `protoc` from 3.15.7 to 3.15.8
- [Release notes](https://github.com/protocolbuffers/protobuf/releases)
- [Changelog](https://github.com/protocolbuffers/protobuf/blob/master/generate_changelog.py)
- [Commits](protocolbuffers/protobuf@v3.15.7...v3.15.8)

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

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

adapter-protobuf-java/build.gradle.kts

@@ -12,7 +12,7 @@ plugins {
 
 project.description = "Defines protobuf messages used in api"
 
-val protobufVersion = "3.15.7"
+val protobufVersion = "3.15.8"
 
 dependencies {
     api("com.google.protobuf:protobuf-java:$protobufVersion")