Backport of add feature flag to gate default Sidekiq concurrency limit calculation

What does this MR do and why?

Backport of !230713 (merged)

Adds a new sidekiq_concurrency_limit_default_calculation ops feature flag (disabled by default) that gates the percentage-based default concurrency limit calculation in calculate_default_limit_from_max_percentage.

In GitLab 18.9, GITLAB_SIDEKIQ_MAX_REPLICAS became set by default in the Helm charts (charts/gitlab!4348), which caused the concurrency limiter to activate on Self-Managed and Dedicated and the concurrency limiter was not tuned for these environments.

This feature flag ensures:

1. Self-Managed and Dedicated installations are not affected by the default concurrency limit calculation unless explicitly opted in
2. GitLab.com continues using the concurrency limiter as-is (flag enabled)
3. Workers with an explicit concurrency_limit lambda are unaffected — only the default calculation is gated

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.