Selective merge train concurrency via feature flag

What does this MR do?

Merge Train works with the default concurrency, 4. However, this is not the best parallel factor for www-gitlab-com project (which we're dogfooding this feature currently) as the project usually has more merge requests on train. This causes a usability issue that users are not seeing MRs have not been merged for a while (e.g. 40 minutes, See more #31692 (comment 235183039))

We should allow this parallel factor configurable in #31692 (closed), but in the meantime, we should allow www-gitlab-com to set a higher concurrency to unblock their daily work.

Related: #31692 (closed)

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• [-] Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team