Fix Import/Export labels error

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/24257

For existing labels that have already been created for merge requests or issues, we should avoid keeping the priorities in the hash as they have also been created already and it will throw an error trying to update the label that already has

• Changelog entry added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together