Update Protobuf, fix publishing

[armiol]

This changeset performs two operations:

1. Updates the Protobuf library version since the one previously used has some recently discovered vulnerabilities. Also, it migrates the project to the latest base library, which also has its Protobuf dependency updated.

2. Excludes all transitive dependencies onto stale io.spine:spine-validate artifact, which are somehow brought via the project dependencies. It caused this PR to fail due to the version conflicts with this artifact.

3. Fixes (and changes) the publishing of Validation modules. There is now two fat JAR artifacts published:

• io.spine.validation:spine-validation-java-extensions — ProtoData extensions allowing to generate the Validation code in Java;
• io.spine.validation:spine-validation-java-runtime which is a runtime library required for the generated Java code to work.

Please note that both of the published fat JARs have NO classifier. It has been removed for simplicity.

The library version has been set to 2.0.0-SNAPSHOT.29.

[codecov[bot]]

Codecov Report

Merging #47 (149e6af) into master (8d15034) will decrease coverage by 0.42%. The diff coverage is n/a.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #47 +/- ## ============================================ - Coverage 45.54% 45.11% -0.43% + Complexity 456 451 -5 ============================================ Files 117 117 Lines 2580 2580 Branches 210 210 ============================================ - Hits 1175 1164 -11 - Misses 1360 1369 +9 - Partials 45 47 +2 ```

[armiol]

@alexander-yevsyukov PTAL.

Hopefully, once this is merged mc-java will become buildable again. I will let you know once I am done with mc-java's branch.

[alexander-yevsyukov]

@armiol, can we have the 2nd artifact name shorter, like spine-validation-java-runtime?