schemadiff: Clean up MySQL version from diff hints

[dbussink]

We already now have the MySQL version in the schemadiff environment, so we should use it from there. Having it also in the diff hints is duplicated and confusing this way.

Related Issue(s)

This should have been cleaned up in #14994 but was overlooked. I'd like to also backport this to v19 as well for consistency reasons since we ship that refactor there and this should have been included in that. Otherwise we have v19 with this slight difference again compared to v20.

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]

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[shlomi-noach]

Nice!

[codecov]

Codecov Report

Attention: 80 lines in your changes are missing coverage. Please review.

Comparison is base (eddb39e) 47.29% compared to head (47969ac) 67.34%.
Report is 137 commits behind head on main.

Files	Patch %	Lines
go/cmd/vtcombo/cli/main.go	0.00%	17 Missing ⚠️
go/cmd/vttablet/cli/cli.go	0.00%	13 Missing ⚠️
go/cmd/vtgate/cli/cli.go	0.00%	8 Missing ⚠️
go/mysql/conn.go	53.84%	6 Missing ⚠️
go/mysql/capabilities/capability.go	91.48%	4 Missing ⚠️
go/mysql/query.go	77.77%	4 Missing ⚠️
go/cmd/vtadmin/main.go	0.00%	3 Missing ⚠️
go/cmd/vtctl/vtctl.go	0.00%	3 Missing ⚠️
go/event/syslogger/syslogger.go	57.14%	3 Missing ⚠️
go/cmd/vtctld/cli/cli.go	0.00%	2 Missing ⚠️
... and 14 more

Additional details and impacted files

@@             Coverage Diff             @@
##             main   #15210       +/-   ##
===========================================
+ Coverage   47.29%   67.34%   +20.04%     
===========================================
  Files        1137     1560      +423     
  Lines      238684   192510    -46174     
===========================================
+ Hits       112895   129642    +16747     
+ Misses     117168    62868    -54300     
+ Partials     8621        0     -8621     

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