Move all OPT_IN_TEST_* jobs to be manually triggered

In the interests of saving time, CO2, and DA $$, what do y'all think about changing the default gitlab_templates pipelines so that all of the OPT_IN_TEST_*  jobs must be triggered manually instead of running automatically? So by default, all pipelines only run a single branch of core, but if maintainers opt in to other testing, it's easy to manually trigger on "exotic" versions of core when an MR is getting close to merge-able?

I just did it manually for one of my contrib projects: https://www.drupal.org/project/pathologic/issues/3579376 but it'd be nice not to have to go out of my way for that.

Tasks remaining

• Use a new set of variables with text-based values 'manual' or 'always',
• Implement the rule for six Drupal Core variants
• Implement for Drupal CMS
• Change variables.yml
• Restrict to a valid set of strings
• Replicate in the Drupal7 pipeline definition
• Update documentation