Deleting an Actions run (UI, API or `RUN_RETENTION_DAYS`) left its
commit statuses linking to the deleted run, so "Details" gave a 404.
`DeleteRun` now clears the `target_url` of the run's statuses and of the
commit's status summary on the run's commit, in the same transaction.
States are kept, so required status checks and merging are unaffected.
This differs from GitHub, which deletes the checks together with the
run.
Fixes https://github.com/go-gitea/gitea/issues/40079
Co-authored-by: silverwind <me@silverwind.io>