Enable retry on transient "Commit not found" errors

Summary

Convert the ci_create_pipeline_worker_retry_on_commit_not_found feature flag from gitlab_com_derisk to beta type and enable it by default.

The flag makes CreatePipelineWorker retry when pipeline creation fails with a transient Gitaly "Commit not found" error (a read-after-write consistency miss). Previously such pipelines were silently dropped with no pipeline object, no user-facing error, and no Sidekiq retry. With the flag enabled, these push/merge pipelines self-heal via the existing Sidekiq retry machinery instead of vanishing.

Related:

• Feature issue: #611125 (closed)
• Rollout issue: #611223
• Introducing MR: !249339 (merged)