fix: don't drop unexpected indexes in Forgejo startup (#9796)

In response to the issue #9755, @Gusted had manually created an index on `action_task` in preparation for the v13 migration of Codeberg.  The restart of v13 dropped the index because `Sync`'s default behaviour is to do so.  I'm inclined to think this doesn't make sense...

Manually tested:
- Confirmed that index with `IDX_...` prefix will be dropped if present and unexpected
- Confirmed that index is not dropped w/ this patch

## Checklist

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### Tests

- I added test coverage for Go changes...
  - [ ] in their respective `*_test.go` for unit tests.
  - [ ] in the `tests/integration` directory if it involves interactions with a live Forgejo server.
- I added test coverage for JavaScript changes...
  - [ ] in `web_src/js/*.test.js` if it can be unit tested.
  - [ ] in `tests/e2e/*.test.e2e.js` if it requires interactions with a live Forgejo server (see also the [developer guide for JavaScript testing](https://codeberg.org/forgejo/forgejo/src/branch/forgejo/tests/e2e/README.md#end-to-end-tests)).

### Documentation

- [ ] I created a pull request [to the documentation](https://codeberg.org/forgejo/docs) to explain to Forgejo users how to use this change.
- [x] I did not document these changes and I do not expect someone else to do it.

### Release notes

- [ ] I do not want this change to show in the release notes.
- [x] I want the title to show in the release notes with a link to this pull request.
- [ ] I want the content of the `release-notes/<pull request number>.md` to be be used for the release notes instead of the title.

Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/9796
Reviewed-by: Gusted <gusted@noreply.codeberg.org>
Co-authored-by: Mathieu Fenniak <mathieu@fenniak.net>
Co-committed-by: Mathieu Fenniak <mathieu@fenniak.net>

models/db/engine.go

@@ -167,6 +167,7 @@ func SyncAllTables() error {
 	}
 	_, err = x.StoreEngine("InnoDB").SyncWithOptions(xorm.SyncOptions{
 		WarnIfDatabaseColumnMissed: true,
+		IgnoreDropIndices:          true,
 	}, sortedTables...)
 	return err
 }