Restore tested releases and update ScalaPB and common protos

[thesamet]

Restore tested publishing for common-protos and bring the libraries up to current upstream releases. The previous dependency refresh was never released, and the workflow did not run on PRs or provide the ci-passed check required by Mergify.

• Build ScalaPB 0.11.21 and 1.0.0-alpha.6 on Scala 2.12.21, 2.13.18, and 3.3.8. Freeze the existing 0.10 artifacts: its generator rejects proto3 optional fields in current Google schemas.
• Update Google common protos to 2.78.0, Pub/Sub to 1.157.0, and PGV to 1.3.3. Incorporate the Dataproc contribution from Add Google Cloud Dataproc protos #166 using 4.96.0 and the maintained ScalaPB variants.
• Add an independent consumer build that imports all four libraries and tests custom options, serialization, Java conversions, and unknown fields. Check packaged generator options for every artifact.
• Gate snapshots and releases on the full compatibility matrix, validate library-specific release tags, and require published common-protos dependencies before Pub/Sub/Dataproc releases. Use Central Portal publishing, current pinned Actions, and JDK 17.
• Document supported versions, the protobuf-java 4 dependency, local verification, and the release process. Group Google updates in Scala Steward and maintain Actions with Dependabot.

Validation: all 24 library variants compile and package; all six independent consumer configurations pass four runtime tests each; four release-selection tests, actionlint, and build formatting checks pass.

This supersedes the open dependency-update backlog (#81, #116, #120, #126, #129, #144, #150, #151, #160, #169, #184, #198, #223, #237, #254, #258, #262, #266, #268). Dataproc implementation credit: Alexis BRENON (#166).