CI: Automate final release workflow with version bumping

Introduce a new CI job in .gitlab-ci-release.yml to automate the
creation of project releases.

This change implements two new jobs in .gitlab-ci-release.yml:

1. A version:bump job that runs on a schedule. On the first Monday of an
even-numbered month, it automatically updates the VERSION and PATCHLEVEL
in the Makefile and pushes the change to the master branch.

2. A release:create job that is triggered by the version bump commit. It
creates the final GitLab Release and corresponding Git tag on the commit
containing the updated Makefile.

This ensures that the repository's version is correctly updated and
committed just before the official release tag is applied, creating a
clean and reliable release history.

This single job, designed to run on a schedule, contains logic to:

  - create a final release (e.g. 2025.08) on the first Monday of an
    even-numbered month.
  - create a release candidate (e.g. 2025.07-rc1) on all other
    scheduled days.

This uses official release-cli to create both the git tag and the
corresponding GitLab Release entry automatically.

Signed-off-by: Simon Glass <sjg@chromium.org>

.gitlab-ci.yml

@@ -27,6 +27,7 @@ stages:
   - test.py
   - sjg-lab
   - world build
+  - version_bump
   - release
 
 .buildman_and_testpy_template: &buildman_and_testpy_dfn