Add gitaly gem bump danger check

This is an alternative corrective action for gitlab-com/gl-infra/production#4903 (closed) that is less risky than gitlab-com/gl-infra/delivery#1821 (closed)

What does this MR do?

Since we now take 5% of traffic to the Canary stage, if there is new functionality deployed in Rails (like a new GRPC) that is not behind a feature flag, it is possible for errors to occur while Canary takes traffic.

The initial proposal was to enforce the following deployment order: gitaly -> praefect -> rails cny -> rails gprd.

This tells us that a couple of things:

• rails is forward compatible, but not backward compatible (it can handle newer gitaly but not older ones)
• gitaly is backward compatible, but not forward compatible (it can handle an older rails/prefect but not a newer one)
• prefect has two interfaces:
  • with rails is backward compatible, but not forward compatible (it can handle an older rails but not a newer one)
  • with gitaly is forward compatible, but not backward compatible (it can handle newer gitaly but not older ones)

Can we safely identify when we are breaking the interface?

The gitaly/praefect interface is already safe (for now) by deploying gitaly first, then preafect. We are only left with the rails interface.

This interface only breaks when we bump the gitaly gem on rails. This is a clear breaking point and we can gate this during the merge request review.

This merge request implements a danger bot rule that informs us to the gitaly gem bumping MR cannot be merged before that gitaly interface is deployed in production.

Screenshots (strongly suggested)

From !65330 (closed)

Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
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• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• I have tested this MR in all supported browsers, or it's not needed.
• I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.

Security

Does this MR contain changes to processing or storing of credentials or tokens, authorization and authentication methods or other items described in the security review guidelines? If not, then delete this Security section.

• 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