Backport: Pin MySQL at 8.0.29 for upgrade/downgrade manual backup test

[mattlord]

Description

This backports the workflow changes from #10847 so that the workflow passes. We do not currently plan to backport the backup fixes past 14.0 and will need to update the website docs that Vitess backups of 8.0.30 are not supported in 12.0 and 13.0.

Related Issue(s)

• Follow-up to and partial cherry-pick of: Backups: Support InnoDB Redo Log Location With 8.0.30+ #10847

Checklist

• "Backport me!" label has been added if this change should be backported
• Tests were added or are not required
• Documentation: Document MySQL 8.0.30+ backup limitations website#1129

[vitess-bot]

Review Checklist

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General

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