Backport GitLab CI to earlier branches

Add .gitlab-ci.yml file to earliest supported branch to enable
automated building and testing for all MariaDB major branches.

For 10.4 we use the bundled SSL to build MariaDB when the platform
does not have OpenSSL 1.1 available. This requires the installation of
gnutls-devel as a dependency of MariaDB Connector/C.
OpenSSL 3.0 support was backported to MariaDB 10.5
(see https://github.com/MariaDB/server/pull/2036, f0fa40ef, 8a9c1e9c)

All new code of the whole pull request, including one or several files
that are either new files or modified ones, are contributed under the
BSD-new license. I am contributing on behalf of my employer
Amazon Web Services, Inc.