Enforce active jobs limit earlier in pipeline creation

What does this MR do and why?

Related to https://gitlab.com/gitlab-org/gitlab/-/issues/356310 (main issue)

Related to https://gitlab.com/gitlab-org/gitlab/-/issues/373284 (FF rollout)

This MR removes the feature flag ci_limit_active_jobs_early which moves the check for active jobs limit earlier in the pipeline creation chain.

The FF was previously added in !97814 (merged).

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