[release-22.0] Fix vtgate upgrade downgrade tests

[arthurschreiber]

Description

This fixes a failing end to end test case on the release-22.0 branch, that's caused by recent CLI argument changes (_ being changed to -) in main.

I didn't bother pulling in the GetFlagVariantForTestsByVersion helper from main, because just using an if/else seemed simple enough here.

Related Issue(s)

Checklist

• "Backport to:" labels have been added if this change should be back-ported to release branches
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• 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

Deployment Notes

[vitess-bot]

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 67.53%. Comparing base (3a28c0c) to head (7e7eccd).
⚠️ Report is 2 commits behind head on release-22.0.

Additional details and impacted files

@@               Coverage Diff                @@
##           release-22.0   #18640      +/-   ##
================================================
- Coverage         67.54%   67.53%   -0.01%     
================================================
  Files              1600     1600              
  Lines            262022   262016       -6     
================================================
- Hits             176976   176951      -25     
- Misses            85046    85065      +19     

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[arthurschreiber]

This is superseded by #18642.