Deprecate and rename some variables that were wrongly named

Issue information

From user: fjgarlin

Related to !180 (merged)

Contribution record

Fork management

Problem/Motivation

Variables that contain open-ended tweaks for jobs and that are likely to be changed by maintainers should start with _ (underscore).

From #3343522: Hide variables that should not be overridden, we have:

In the variables file: https://git.drupalcode.org/project/gitlab_templates/-/blob/main/includes/include.drupalci.variables.yml?ref_type=heads
We have:
- First group which starts with underscore for probably the most common tweaks for the jobs like Drupal version, php version, database type...
- Then the SKIP_ ones, which are switches (on/off)
- Then the OPT_IN_ ones, which are switches (on/off)
- Then LENIENT_ALLOW_LIST and COMPOSER_PATCHES_FILE which should probably start with _ (as it is configuration for the composer jobs) but don't <=== I only see an inconsistency in here.

Then a whole bunch of variables that we don't want changes to:
- _CONFIG_DOCKERHUB_ROOT
- CORE_ variables
- Database related variables
- Some constraints

And finally, variables to test downstream and merge requests or fixed tags.

So at this point, I only think we have a couple of variables that don't follow the pattern. We could do a deprecation job for them or just live with them as they are.

Proposed resolution

We can reuse the .pre job we had for SKIP_COMPOSER and add the patches+lenient variables there (through the child issue).

Remaining tasks

MR and also do a gitlab search of usage of these variables to let the maintainers know about it.
Lenient search.
Composer patches search.