Add downgrade cancellation e2e tests

[henrybear327]

Part 2 of the e2e test - downgrade cancellation is called after the cluster has been partially downgraded

This PR is currently based on #19244, thus, it's put in draft mode for now.

Reference: #17976

Please read https://github.com/etcd-io/etcd/blob/main/CONTRIBUTING.md#contribution-flow.

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 68.88%. Comparing base (532c601) to head (7afa927).

Additional details and impacted files

see 20 files with indirect coverage changes

@@            Coverage Diff             @@
##             main   #19252      +/-   ##
==========================================
+ Coverage   68.87%   68.88%   +0.01%     
==========================================
  Files         420      420              
  Lines       35680    35680              
==========================================
+ Hits        24573    24579       +6     
+ Misses       9685     9679       -6     
  Partials     1422     1422              

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[henrybear327]

Will continue to investigate this tomorrow