e2e: use different QPS for BackendUpgrade test

[zirain]

xref: #6035

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 70.78%. Comparing base (b01c373) to head (8a508d5).
Report is 11 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6373      +/-   ##
==========================================
+ Coverage   70.76%   70.78%   +0.02%     
==========================================
  Files         220      220              
  Lines       37089    37089              
==========================================
+ Hits        26246    26254       +8     
+ Misses       9306     9299       -7     
+ Partials     1537     1536       -1     

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