Backport of 'Split refresh worker into new workers'

What does this MR do and why?

This MR backports the enablement of three feature flags that split the merge request refresh worker into specialized workers for better performance and maintainability.

The changes promote three feature flags from wip to beta status and enable them by default:

• split_refresh_approval_worker - Handles approval-related updates in a dedicated worker
• split_refresh_worker_pipeline - Manages pipeline-related refresh operations separately
• split_refresh_worker_web_hooks - Processes webhook triggers independently

Why this is needed in 18.5:

The defaulting the split of the refresh worker was merged in 18.6 (!211874 (merged)) to improve performance by distributing the workload across specialized workers instead of having one monolithic refresh worker. This backport ensures the 18.5 stable branch has these performance improvements enabled by default, matching the current state on the default branch.

This addresses a performance issue where the single refresh worker could become a bottleneck when processing merge request updates.

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).
• The MR title is descriptive (e.g. "Backport of 'title of default branch MR'"). This is important, since the title will be copied to the patch blog post.
• Required labels have been applied to this merge request
  • severity label and bug subtype labels (if applicable)
  • If this MR fixes a bug that affects customers, the customer label has been applied.
• This MR has been approved by a maintainer (only one approval is required).
• Ensure the e2e:test-on-omnibus-ee job has succeeded, or if it has failed, investigate the failures. If you determine the failures are unrelated, you may proceed. If you need assistance investigating, reach out to a Software Engineer in Test in #s_developer_experience.

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).
• Once the backport has been merged, the commit changes will be automatically deployed to a release environment that can be used for manual validation. See after merging runbook for details.