Fix release_branch_regexp (#59590)
Apparently the non-capturing group was incorrect.
I verified the logic in a branch in grafana-github-actions:
https://github.com/grafana/grafana-github-actions/compare/jdb/2022-11-prove-grafana-regexp-behaves-correctly?expand=1
Signed-off-by: Jack Baldry <jack.baldry@grafana.com>
Signed-off-by: Jack Baldry <jack.baldry@grafana.com>
(cherry picked from commit a343defe64)
Co-authored-by: Jack Baldry <jack.baldry@grafana.com>