Resolve "Re-enable the `RSpec/SingleLineHook` cop"

What does this MR do?

1. This updates gitlab-styles to 2.3.1 which fixes the RSpec cops to only check for test files.
2. This re-enables the RSpec/SingleLineHook cop since it had been previously automatically disabled because of false-positive offenses found in non-test files (due to the bug described in 1. and fixed by gitlab-styles 2.3.1.

Are there points in the code the reviewer needs to double check?

I don't think so.

Why was this MR needed?

Because the RSpec/SingleLineHook cop had been erroneously disabled.

Does this MR meet the acceptance criteria?

• Review
  • Has been reviewed by Backend
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?

Closes #42385 (closed)