Treat ClusterVersionSet as 3.5 version

[ahrtr]

Link to #19168

[ahrtr]

The test failures are expected. I will fix them later.

@serathius PTAL

[ahrtr]

cc @fuweid @siyuanfoundation

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 68.79%. Comparing base (32cfd45) to head (aef91b8).
Report is 1 commits behind head on main.

Additional details and impacted files

Files with missing lines	Coverage Δ
server/storage/wal/version.go	51.53% <100.00%> (ø)

... and 27 files with indirect coverage changes

@@            Coverage Diff             @@
##             main   #19263      +/-   ##
==========================================
- Coverage   68.83%   68.79%   -0.05%     
==========================================
  Files         420      420              
  Lines       35678    35681       +3     
==========================================
- Hits        24560    24546      -14     
- Misses       9688     9713      +25     
+ Partials     1430     1422       -8     

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[siyuanfoundation]

This would probably not impact 3.6->3.5. But for 3.7->3.6 downgrade with offline migration, if there is an entry of ClusterVersionSet to be applied after the downgrade, would that mean the cluster version be set to 3.7 when the old ClusterVersionSet entry is applied?

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