Document MySQL 8.0.30+ backup limitations

[mattlord]

You can see the changes from here:

• https://deploy-preview-1129--vitess.netlify.app/docs/12.0/user-guides/operating-vitess/backup-and-restore/

You can use the left hand navigation to see the 3 backup and restore pages. You can change the version number in the URL to see the changes for 13.0 and 14.0 (15.0 needs none as it will support 8.0.30 in all releases).

[vitess-bot]

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