[#325] Upgrade Central publishing plugin and remove Maven 3.9 pin (#1278)

Fixes OpenIdentityPlatform/commons#325.

Follows #1187 and OpenIdentityPlatform/commons#388.

Upgrade `org.sonatype.central:central-publishing-maven-plugin` from
0.11.0 to 0.12.0 and remove the Maven 3.9.16 installation step
introduced by #1187. Releases use the runner's Maven again.

Version 0.12.0 removes the local-repository metadata, tracking files,
and locks that Maven 3.10 writes into the staging directory, preventing
Central's “Bundle has content that does NOT have a .pom file” rejection.
See the [official release
notes](https://central.sonatype.org/publish/publish-portal-maven/#0120-13-oct-2026)
and [upstream
fix](https://github.com/sonatype-nexus-community/central-publishing-maven-plugin/pull/3).

Validation:
- POM XML and workflow YAML parse; `git diff --check` passes.
- Parent POM staging with Maven 3.10.0 / JDK 11, using a temporary
release-version copy, dummy credentials, and `deploy -N
-DskipUpload=true` completed successfully. The bundle contains exactly
the parent POM and its four checksums, with no Maven metadata, tracking
files, or locks. Nothing was uploaded to Central.

The full reactor and actual release workflow were not run.