Set ActorTableSchemaMigrationStage => OLD on test wikis, mw.org

After finding some performance problems, I'm going to look at doing a
"write-both read-old" → "write-both read-new" progression rather than
"write-both read-both" → "write-new read-both" as originally planned.
The necessary code for read-both is already complicated, and doing it in
special pages is much more so.

The risk is more chance of user-obvious breakage once we do try to do
the switch from read-old to read-new, but that's probably acceptable.

Bug: T188327
Bug: T204669
Change-Id: Idd8a6c05a3fb506578632ce0287058805d88ccd9