Remove at_most helper on ApplicationRecord

What does this MR do?

Closes #321634 (closed)

at_most is just an alias / wrapper around limit. It was apparently used to silence/circumvent a lint error.

We decided to instead allow limit outside of ActiveRecord objects: https://gitlab.com/gitlab-org/gitlab-styles/-/merge_requests/71

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