Put allow_composite_identities_to_run_pipelines behind ff

What does this MR do and why?

This MR backports a fix that didn't get into 17.11 release: Put allow_composite_identities_to_run_pipelines... (!187953 - merged).

It disables the functionality that allows running CI pipelines on MRs created by composite identities only when the flag is explicitly enabled: https://gitlab.com/gitlab-org/gitlab/-/issues/512509+.

After the testing, it was identified that the behavior is disruptive and unintuitive and now jeopardizes the successful release of Amazon Q functionality, which can be considered S1/S2 bug.

The simplest way to fix it is to put the actual check behind a feature flag. The original MR includes a database migration and was also backported to older releases: https://gitlab.com/gitlab-org/security/gitlab/-/merge_requests/4801+, so just reverting it is a more complicated option.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• This MR is backporting a bug fix, documentation update, or spec fix, previously merged in the default branch.
• The MR that fixed the bug on the default branch has been deployed to GitLab.com (not applicable for documentation or spec changes).
• This MR has a severity label assigned (if applicable).
• Set the milestone of the merge request to match the target backport branch version.
• This MR has been approved by a maintainer (only one approval is required).
• Ensure the e2e:test-on-omnibus-ee job has either succeeded or been approved by a Software Engineer in Test.

Note to the merge request author and maintainer

If you have questions about the patch release process, please:

• Refer to the patch release runbook for engineers and maintainers for guidance.
• Ask questions on the #releases Slack channel (internal only).