Throttle /api/v4/groups/:id/projects endpoint

What does this MR do?

We determined in https://gitlab.com/gitlab-com/gl-infra/infrastructure/-/issues/9420 that we want to apply stricter rate limits to /api/v4/groups/:id/projects.

This patch introduces that rate limit.

We cannot use the existing protected paths feature for two reasons:

• It does not support regexp
• It only matches on POST requests

I was not comfortable making either of those two changes to the mechanism, as it could change the behaviour in unintended ways.

Screenshots

n/a

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• 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
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

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

cc @mwasilewski-gitlab @msmiley @stanhu @engwan @ahmadsherif