Execute pipelines on `main` branch

gitlab-com/gl-infra/delivery#1357 (closed) plans to rename gitlab-org/gitlab default branch from master to main. The timeline is still in the works, tentative date could be after %14.0

Broad plan to ease the master to main transition is to do it gradually:

1. First, we mirror commits from master to main
2. Then, we change the default branch to main, and,
3. We flip the mirror back, commits will be mirrored from main to master

Note: There are more details involved in the default branch renaming, see gitlab-com/gl-infra/delivery#1357 (closed) for further details.

For this transition to work, pipelines need to be executed on main branch, just like we currently do in master, stable and auto-deploy branches.

Scope of change

• Add pipeline rules for main !54409 (merged)
• Create new pipeline schedules for main
• Update CI_PRE_CLONE_SCRIPT and cache-repo scripts to be compatible with main
• Update hard-coded references to master in scripts
• ...