rm priority field from EnvoyExtensionPolicy design doc

[arkodg]

Fixes: #6552

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 71.11%. Comparing base (88cec5a) to head (49d9615).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6553      +/-   ##
==========================================
+ Coverage   71.09%   71.11%   +0.02%     
==========================================
  Files         220      220              
  Lines       37892    37892              
==========================================
+ Hits        26938    26948      +10     
+ Misses       9383     9374       -9     
+ Partials     1571     1570       -1     

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