Bump protobuf-java and protobuf-java-util from 4.31.1 to 4.33.1 (#18)

* Bump protobuf-java and protobuf-java-util from 4.31.1 to 4.33.1

Restores backward compatibility with protobuf v3/v4 generated code.
This matches the upgrade done in rules_kotlin to address compatibility
issues with Kotlin annotation processors like KSP that need to load
protobufs generated with both v3 and v4 protoc.

Changes:
- Updated protobuf Bazel module version from 31.1 to 33.1
- Updated protobuf-java from 4.31.1 to 4.33.1
- Updated protobuf-java-util from 4.31.1 to 4.33.1
- Added proto_version variable to keep versions in sync

References:
- https://github.com/protocolbuffers/protobuf/releases/tag/v33.1
- https://github.com/protocolbuffers/protobuf/releases/tag/v33.0
- https://protobuf.dev/support/cross-version-runtime-guarantee/#backwards

* Add pin file to .gitignore

The rules_jvm_external++maven+bazel_worker_maven_install.json file is
auto-generated by Bazel's bzlmod system and doesn't need to be checked in.