[test] Correct the expected version in downgrade test case

[ahrtr]

Fix #19391

There are two issues in the existing downgrade e2e test cases:

• as long as it's downgrade is enabled, then we should expect a lower cluster version.
• after we upgrade all member to its original version, we should expect a higher cluster version.

Thanks @fuweid for the analysis.

cc @siyuanfoundation @serathius @henrybear327

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 68.92%. Comparing base (c0e7e8c) to head (57dc6f3).
Report is 2 commits behind head on main.

Additional details and impacted files

see 22 files with indirect coverage changes

@@            Coverage Diff             @@
##             main   #19398      +/-   ##
==========================================
- Coverage   68.95%   68.92%   -0.03%     
==========================================
  Files         420      420              
  Lines       35759    35764       +5     
==========================================
- Hits        24656    24652       -4     
  Misses       9683     9683              
- Partials     1420     1429       +9     

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[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: ahrtr, fuweid, henrybear327

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[ahrtr]

/cherry-pick release-3.6

[k8s-infra-cherrypick-robot]

@ahrtr: new pull request created: #19400

In response to this:

/cherry-pick release-3.6

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