Match only prefix so we don't match on /spec/ for our CODEOWNER

What does this MR do and why?

Match only prefix so we don't match on /spec/ for our CODEOWNER

Following !84141 (merged) and don't match on spec/lib/gitlab/background_migration/reset_duplicate_ci_runners_token_encrypted_values_on_projects_spec.rb as shown in !82309 (comment 905952812)

This was reported at https://gitlab.com/gitlab-org/gitlab/-/issues/357296#note_905950231

Screenshots or screen recordings

These are strongly recommended to assist reviewers and reduce the time to merge your change.

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.