chore: clean up gatewayapi v1alpha2 helpers

[shawnh2]

What type of PR is this?

What this PR does / why we need it:

gwapiv1a2.BackendRef is an alias for gwapiv1.BackendRef

Which issue(s) this PR fixes:

Fixes #

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 3 lines in your changes missing coverage. Please review.

Project coverage is 65.84%. Comparing base (26ec49a) to head (8c793b3).
Report is 8 commits behind head on main.

Files with missing lines	Patch %	Lines
internal/provider/kubernetes/routes.go	0.00%	0 Missing and 3 partials ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4482   +/-   ##
=======================================
  Coverage   65.83%   65.84%           
=======================================
  Files         203      202    -1     
  Lines       30699    30678   -21     
=======================================
- Hits        20212    20200   -12     
+ Misses       9335     9323   -12     
- Partials     1152     1155    +3     

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[shawnh2]

/retest