fix: remove backendrefs validation

[zhaohuabing]

Fixes #4704

Remove backendrefs validation from the ExtAuth API for backward compatiblity.

Release Notes: Yes

[arkodg]

hey @zhaohuabing thanks for picking this up, can we add a test in gateway-api to make sure the backendRef field translation actually works ?

cc @zirain

[codecov]

Codecov Report

Attention: Patch coverage is 80.55556% with 7 lines in your changes missing coverage. Please review.

Project coverage is 65.58%. Comparing base (c2b0ee3) to head (c42636a).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
internal/gatewayapi/securitypolicy.go	80.55%	6 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4705      +/-   ##
==========================================
+ Coverage   65.55%   65.58%   +0.03%     
==========================================
  Files         211      211              
  Lines       31972    31995      +23     
==========================================
+ Hits        20960    20985      +25     
+ Misses       9768     9767       -1     
+ Partials     1244     1243       -1     

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