ci: limiting mirroring concurrency

## Problem
Mirroring actions are failing because other actions have already mirrored what
it was about to mirror.

E.g.

* https://github.com/DeveloperC286/starlingbanktechnicalchallenge/actions/runs/7639729071/job/20813352947

```
Run git push --mirror "***gitlab.com/DeveloperC/${REPOSITORY_NAME}"
warning: redirecting to https://gitlab.com/DeveloperC/starlingbanktechnicalchallenge.git/
remote: error: cannot lock ref 'refs/heads/earthly_upgrade': unable to resolve reference 'refs/heads/earthly_upgrade'
remote: error: cannot lock ref 'refs/pull/92/head': reference already exists
To https://gitlab.com/DeveloperC/starlingbanktechnicalchallenge
   f4c0fa2..dfc1c32  main -> main
 ! [remote rejected] earthly_upgrade (failed to update ref)
 ! [remote rejected] refs/pull/92/head -> refs/pull/92/head (failed to update ref)
error: failed to push some refs to 'https://gitlab.com/DeveloperC/starlingbanktechnicalchallenge'
```

* https://github.com/DeveloperC286/starlingbanktechnicalchallenge/actions/runs/7639728988/job/20813352607

```
Run git push --mirror "***gitlab.com/DeveloperC/${REPOSITORY_NAME}"
warning: redirecting to https://gitlab.com/DeveloperC/starlingbanktechnicalchallenge.git/
remote: error: cannot lock ref 'refs/heads/main': is at dfc1c32 but expected f4c0fa2
To https://gitlab.com/DeveloperC/starlingbanktechnicalchallenge
 - [deleted]         earthly_upgrade
 * [new reference]   refs/pull/92/head -> refs/pull/92/head
 ! [remote rejected] main -> main (failed to update ref)
error: failed to push some refs to 'https://gitlab.com/DeveloperC/starlingbanktechnicalchallenge'
```

## Solution
So we are limiting the concurrency to prevent this.

## Related
* https://docs.github.com/en/actions/using-jobs/using-concurrency
* #85

.github/workflows/mirroring.yml

@@ -2,6 +2,10 @@ name: Mirroring
 
 on: [push, delete]
 
+concurrency:
+  group: ${{ github.workflow }}
+  cancel-in-progress: true
+
 jobs:
   GitLab:
     runs-on: ubuntu-latest