Deprecate legacy preference upgrade mechanism

[david-allison]

That should probably be added in a comment here in the code so future developers are not lured in, and/or could use a blurb in developer guide "how to do a preferences upgrade"...

Originally posted by @mikehardy in https://github.com/ankidroid/Anki-Android/pull/11555#discussion_r890150791

[github-actions[bot]]

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[BrayanDSO]

There is a comment there IIRC