Update MySQL 8.4 to be the default

[dbussink]

This is for the upcoming Vitess 23, so that we ship it with MySQL 8.4 by default when it releases, as otherwise MySQL 8.0 will go EOL for a significant part of v23's lifetime.

Once this is all good and working, we also need the changes in the release notes of course.

Related Issue(s)

Part of #16466

Checklist

• "Backport to:" labels have been added if this change should be back-ported to release branches
• If this change is to be back-ported to previous releases, a justification is included in the PR description
• Tests were added or are not required
• Did the new or modified tests pass consistently locally and on CI?
• Documentation was added or is not required

[vitess-bot]

Review Checklist

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[codecov]

Codecov Report

❌ Patch coverage is 75.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 67.50%. Comparing base (104b755) to head (a89eb60).
⚠️ Report is 5 commits behind head on main.

Files with missing lines	Patch %	Lines
go/vt/mysqlctl/mysqld.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##             main   #18569    +/-   ##
========================================
  Coverage   67.49%   67.50%            
========================================
  Files        1607     1607            
  Lines      263144   263325   +181     
========================================
+ Hits       177621   177752   +131     
- Misses      85523    85573    +50     

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[dbussink]

Website docs update in vitessio/website#2002.