setup-java@v4 has no maven-version input, so the previous pin was
silently ignored and the runner kept using Maven 3.10, whose stray
staging files still got bundled and rejected by Central. Add the
Maven Wrapper pinned to 3.9.16 and switch the workflow to ./mvnw.
Maven 3.10 leaves _remote.repositories and maven-metadata files in
the staging dir, which central-publishing-maven-plugin bundles into
the deployment. Central then rejects the release with 'Bundle has
content that does NOT have a .pom file: de/extio/lm-lib'. Pin the
setup-java Maven version until upstream handles 3.10.
