feat: Improve Gitea-compatible forge migrations, fix many migration bugs (#39424)

Supports migrating from Gitea-compatible forges like Forgejo and fixes
many migration bugs.

1. Pre-release and `+gitea-` suffixed server versions no longer drop
reviews, reactions and pagination.
2. Review states, dismissals, scoped and archived labels and merge bases
are migrated.
3. Disabled units, external trackers, unavailable release assets, and
GitLab confidential issues and internal notes are skipped.
4. Credentials are only sent to the source host, and `Retry-After` rate
limits are waited out.
5. GitHub migrations of only issues or pull requests, basic auth clones
and dismissed reviews work again.
6. The API returns 404 for releases of a disabled unit and shows
migrated authors as mannequins, replacing the never populated
`original_author` and `original_author_id` fields.

Fixes: https://github.com/go-gitea/gitea/issues/19533
Fixes: https://github.com/go-gitea/gitea/issues/35885
Fixes: https://github.com/go-gitea/gitea/issues/18275
Fixes: https://github.com/go-gitea/gitea/issues/35069
Fixes: https://github.com/go-gitea/gitea/issues/17763
Fixes: https://github.com/go-gitea/gitea/issues/39548
Co-authored-by: bircni <bircni@icloud.com>

modules/migration/file_format_testdata/issue_a.yml

@@ -8,3 +8,7 @@
   created: 2021-05-27T15:24:13+02:00
   updated: 2021-11-11T10:52:45+01:00
   closed: 2021-11-11T10:52:45+01:00
+  labels:
+    - name: scope/label_a
+      exclusive: true
+      archived: true