[EE] Fix `ee-files-location-check` job

What does this MR do?

This ensures that a file added in a CE MR is not detected as EE-only, as long as the CE branch name is equal to the EE branch minus a leading ee- or trailing -ee.

This can be seen in https://gitlab.com/gitlab-org/gitlab-ee/-/jobs/53259933. Without this fix, the job would have advised to move the new-file-in-ce to ee/, which is wrong.

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

As you can see, the CE MR is adding a new-file-in-ce file. This file is not detected as EE-only, but new-file-in-ee (added in this MR) is correctly.

Why was this MR needed?

Because the script wasn't handling EE-porting MR (as advised in https://docs.gitlab.com/ee/development/automatic_ce_ee_merge.html#avoiding-ce-gt-ee-merge-conflicts-beforehand) correctly. A new file created in CE, would be detected as EE-only in the EE MR counterpart.

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?