Remove the rate_limit_pipeline_cancel feature flag

Daniel Prauserequested to merge
628993-remove-rate-limit-pipeline-cancel-flag into master

What does this MR do and why?

Removes the rate_limit_pipeline_cancel feature flag, so the pipeline cancellation rate limit from !255403 (merged) applies everywhere.

The flag has been enabled for 100% of actors on GitLab.com since 2026-09-24, after a staged rollout from 2026-09-23. In the logs, the limit only throttled two patterns: a user clicking Cancel repeatedly on the same pipeline, and a script cancelling many pipelines in one project in parallel. The per-user-and-pipeline and per-user-and-project limits behaved as intended in both cases.

This removes the flag check in Ci::CancelPipelineService, the flag definition, and the spec context for the disabled flag. The flag was introduced and removed in the same release (19.5), so the flag history entries and the flag note are removed from the docs, as the documentation guidelines describe.

References

Screenshots or screen recordings

Not applicable, backend and docs only.

How to set up and validate locally

bin/rspec spec/services/ci/cancel_pipeline_service_spec.rb

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Closes #628993 (closed)

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