Update database migration docs for 5.3

[michaelsproul]

Proposed Changes

Update the book with the correct schema version (v21) for v5.3.0 and information on the available downgrade.

Additional Info

We could wait to merge this to see whether v5.3.0 falls in July or August, or we could just YOLO and assume the release happens on Thursday or later

[chong-he]

Looks good, putting August is fine imho

[michaelsproul]

@mergify queue

[mergify]

queue

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[jimmygchen]

@mergify requeue

[mergify]

requeue

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[mergify]

queue

✅ The pull request has been merged automatically

The pull request has been merged automatically at 00038da