'group2' doesn't work to specify configuration in operations/mediawiki-config InitialiseSettings.php etc.

I learned about this when I tried doing it in https://gerrit.wikimedia.org/r/c/operations/mediawiki-config/+/901697. CI passed the change, but it had no effect. @taavi noticed that the diffConfig job reported that there are no changes only after we had deployed it and debugged it for 15 minutes.

Our configuration tries to use 'group2' in one more place: https://gerrit.wikimedia.org/g/operations/mediawiki-config/+/bd8a5811ff536e9236638846f68020d45950da50/wmf-config/InitialiseSettings-labs.php#224 (luckily it looks like a no-op placeholder) and I have only recently removed a few more no-op uses: https://gerrit.wikimedia.org/r/c/operations/mediawiki-config/+/854100/2/wmf-config/InitialiseSettings.php so it seems like no one realizes that it's not supported.

This is probably because it's not listed under DB_LISTS here: https://gerrit.wikimedia.org/g/operations/mediawiki-config/+/bd8a5811ff536e9236638846f68020d45950da50/multiversion/MWMultiVersion.php#36 it was removed from this list in https://gerrit.wikimedia.org/r/c/operations/mediawiki-config/+/578655 (2020) by @Krinkle.

In this task, we should either add support for it, or make sure that attempts to use it will be rejected by CI.