fix: corrupted wiki unit default permission (#8234 follow-up) (#8258)

Closes #8119, follow-up of #8234

#8234 "only" fixed the case when a new wiki setting was applied.

However it lacked 2 aspects:
- fixing the already corrupted unit permission in the database (required a migration)
- fixing the API route

Both aspects should now be covered.

Additionally, I commented out the unused `UnitAccessMode` and indicated that they are only used for the wiki-unit (hopefully saving some time to future code-readers).

### Testing

- go to a commit before #8234 (e.g. 285f66b782)
- create 3 repositories
  - save the wiki settings of the second, without any change
  - set the wiki of the third to be globally writable
- verify the `default_permissions` column in the database, of the unit `5`: 0, 2, 3
- stop Forgejo, switch to this PR and start Forgejo
- verify that the logs writes `Migration[35]: Fix wiki unit default permission`
- verify the `default_permissions` column in the database, of the unit `5`: 0, 0, 3

Before:

![2025-06-23-150055.png](/attachments/1eb92507-b8b4-422c-921c-4296a91172e7)

After:

![2025-06-23-150853.png](/attachments/c7b53397-54fe-487d-89da-e10b26538cde)

- [x] I did not document these changes and I do not expect someone else to do it.
- [x] I do not want this change to show in the release notes.

Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/8258
Reviewed-by: Earl Warren <earl-warren@noreply.codeberg.org>
Co-authored-by: oliverpool <git@olivier.pfad.fr>
Co-committed-by: oliverpool <git@olivier.pfad.fr>

routers/api/v1/repo/repo.go

@@ -861,7 +861,7 @@ func updateRepoUnits(ctx *context.APIContext, owner string, repo *repo_model.Rep
 			if *opts.GloballyEditableWiki {
 				wikiPermissions = repo_model.UnitAccessModeWrite
 			} else {
-				wikiPermissions = repo_model.UnitAccessModeRead
+				wikiPermissions = repo_model.UnitAccessModeUnset
 			}
 		}