Use ruby 2.5.8 or 2.7.6 via rbenv for Puppet 7 tests

Use ruby 2.5.8 or 2.7.6 via rbenv for Puppet 7 tests

Install and use rbenv using:

* 2.7.6 with puppet latest (currently 7.24.0)
* 2.5.8 with puppet with CERN current 7.12

Moving to rbenv since ruby is too old on EL8 at 2.5 and
too new on EL9 at 3.0.

Once you also throw puppet8 into that in the future it will want 3.1

We also introduce templates into generated .gitlab-ci.yml to reduce duplication in that file.

The only test change is that a customisation (via .sync.yml) of strict_variables: no on both puppet7 and puppet7_latest could be applied. The existing default setting of strict_variables: yes throws CI errors if you use an undefined variable even though puppet evaluates that in real life as an empty string. At some future date the option to specify strict_variables=false will be removed. Relying on undefined variables is best avoided generally.