Forgot to bump the version when creating the migration. Also convert
the migration to a single sql query, although the migration will never
run because I ran the query manually to fix things up after finding
out the data was "dirty" since the new code (assuming only modern
statuses) was merged without running the migration.
Thankfully it looks like the impact was not too severe (because the
legacy statuses should only be present on very old commits / PRs), I
don't remember when I deployed the update but apparently just a pair
of PRs got affected, because their `previous_failure` was the old
style and thus broke the "new failure" check.
When I updated the status storage (including `previous_failure`) for
some reason I didn't just migrate from the old to the new format, and
added bridge functions instead.
This is not really necessary (or useful), so convert all the legacy
data and remove the conversion helpers.
Relates to #802
