mysqlctl: Improve backup restore compatibility check

[dbussink]

This check has to be smarter to allow certain backup restore scenarios. Starting from MySQL 8.0.35, it's allowed to restore into older versions within the same LTS stream. This is something we didn't allow so far, but we need to for managing the more recent MySQL releases correctly.

This also implies that we don't need upgrade safe flags for these cases as well and allows it even if that flag is off now.

Related Issue(s)

Fixes #15855

Checklist

• "Backport to:" labels have been added if this change should be back-ported to release branches
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• Documentation was added or is not required

[vitess-bot]

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[codecov]

Codecov Report

Attention: Patch coverage is 84.21053% with 3 lines in your changes are missing coverage. Please review.

Project coverage is 68.44%. Comparing base (f118ba2) to head (6472ee0).
Report is 64 commits behind head on main.

Files	Patch %	Lines
go/vt/mysqlctl/backupengine.go	82.35%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #15856      +/-   ##
==========================================
+ Coverage   68.40%   68.44%   +0.03%     
==========================================
  Files        1556     1559       +3     
  Lines      195121   196535    +1414     
==========================================
+ Hits       133479   134519    +1040     
- Misses      61642    62016     +374     

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[dbussink]

See also mysql/mysql-server@9a940ab which encodes in MySQL itself.