Run `*-as-if-foss` jobs on master only

What does this MR do?

This is mostly to reduce CI costs (by cutting the MR pipeline jobs count from 204 to 141).

The `* as-if-foss` jobs allows to run GitLab's test suite "as-if-FOSS", meaning as if the jobs would run in the context
of the `gitlab-org/gitlab-foss` project. These jobs are only created in the following cases:

- `master` commits (pushes and scheduled pipelines).
- `gitlab-org/security/gitlab` merge requests.
- Merge requests which include `RUN AS-IF-FOSS` in their title.
- Merge requests that changes the CI config.

The `* as-if-foss` jobs have the `FOSS_ONLY='1'` variable set and gets their EE-specific
folders removed before the tests start running.

The intent is to ensure that a change won't introduce a failure once the `gitlab-org/gitlab` project will be synced to
the `gitlab-org/gitlab-foss` project.

I have added the above to the documentation.

Based on !31914 (merged).

Does this MR meet the acceptance criteria?

Conformity

• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Separation of EE specific content

Closes #213944 (closed).