Create API/EndpointDeprecation linter to enforce deprecation style guide

Summary

Create a new RuboCop linter API/EndpointDeprecation to enforce the API deprecation style guide.

Background

Following !214743 (merged) and !214914 (merged), we have a clear style guide for marking API endpoints as deprecated using the deprecated: true flag. However, many instances still mention deprecation in descriptions instead of using the proper flag.

Proposal

Create a linter that:

1. Checks both desc and detail for mentions of "deprecated" (patterns like [DEPRECATED]:, [Deprecated], (deprecated), etc.)
2. Ensures the proper deprecated: true flag is used instead
3. Scans endpoint definitions in lib/api

Related

• Related to #581045 (closed)
• Follow-up from !214743 (merged)
• Style guide in !214914 (merged)
• Epic: gitlab-org/quality&231