Merge branch 'fix_epic_boards_with_labels_unification' into 'master'

Fix epic boards lists filtered by labels to include unified labels

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/157461



Merged-by: Tiger Watson <twatson@gitlab.com>
Approved-by: Vitali Tatarintev <vtatarintev@gitlab.com>
Approved-by: Nicolas Dular <ndular@gitlab.com>
Approved-by: Eugenia Grieff <egrieff@gitlab.com>
Approved-by: Tiger Watson <twatson@gitlab.com>
Reviewed-by: Alexandru Croitor <acroitor@gitlab.com>
Reviewed-by: Nicolas Dular <ndular@gitlab.com>
Co-authored-by: Alexandru Croitor <acroitor@gitlab.com>