ci: replace battila7/get-version-action (fix set-output & node deprecation)

[BWibo]

Why

battila7/get-version-action@v2 declares the ancient node12 runtime and internally uses the deprecated set-output workflow command, so it emits both the Node.js 20 and set-output deprecation warnings in the build-push release job:

The set-output command is deprecated and will be disabled soon.
Node.js 20 actions are deprecated. …

All other actions in this repo already run on the Node 24 runtime (checkout@v6, setup-java@v5, gradle/actions/setup-gradle@v6, docker/* v4–v7, short-sha@v4.0), so no version bumps are needed here — this is the only remaining offender.

What

Replace the battila7/get-version-action@v2 step in docker-build-push-release.yml with a small POSIX shell step that derives the version from the release tag and writes to $GITHUB_OUTPUT:

- name: Get release version
  id: release_version
  env:
    REF_NAME: ${{ github.event.release.tag_name || github.ref_name }}
  run: |
    version="${REF_NAME}"
    echo "version=${version}" >> "$GITHUB_OUTPUT"
    echo "version-without-v=${version#v}" >> "$GITHUB_OUTPUT"

The version / version-without-v outputs (the only ones consumed downstream, in the CITYDB_TOOL_VERSION build-arg) are preserved, so the rest of the workflow is unchanged.

Validation

• YAML parsed; actionlint clean.
• Shell logic unit-tested for v1.2.3 / 1.2.3 tag forms → correct version / version-without-v.