Move core apps backgroudjobs to OCP

[rullzer]

Since N15 we finally have an OCP mechanism to handle background jobs. We should use it properly.

[szaimen]

I suppose this is still valid?

[joshtrichards]

Looks like many/most have been moved. Just a little bit remaining:

• Most core background jobs and most core app background jobs were migrated in #33047 (<--good model for anyone looking to pick up any of the below remaining "to do" items for this issue; also the Dev Manual Background jobs (Cron) chapter)
• Remaining ones in server repo appear to be:
  • [ ] OCA\User_LDAP\Migration\UUIDFix
  • [ ] OCA\DAV\Migration\BuildSocialSearchIndexBackgroundJob
  • [ ] OCA\DAV\Migration\BuildCalendarSearchIndexBackgroundJob

Also:

• [ ] A small handful of tests need updating
• [ ] What about the repair background jobs that aren't in apps?
  • OC\Repair\OwnCloud\*
• [ ] Check shipped/default apps that aren't in the server repo: create issues where needed in their respective repos (search string: "OC\BackgroundJob")

Outside of scope, but related:

• [ ] Recommended apps: create issues where needed in their respective repos (search string: "OC\BackgroundJob")
• [ ] Other @nextcloud apps: create issues where needed in their respective repos (search string: "OC\BackgroundJob")