chore: use net.JoinHostPort

[zirain]

fixes: #4597

[codecov]

Codecov Report

Attention: Patch coverage is 83.33333% with 2 lines in your changes missing coverage. Please review.

Project coverage is 65.42%. Comparing base (db68027) to head (9ac2399).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
internal/gatewayapi/securitypolicy.go	80.00%	1 Missing ⚠️
internal/kubernetes/port_forwarder.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4599      +/-   ##
==========================================
+ Coverage   65.40%   65.42%   +0.01%     
==========================================
  Files         211      211              
  Lines       31887    31887              
==========================================
+ Hits        20855    20861       +6     
+ Misses       9785     9781       -4     
+ Partials     1247     1245       -2     

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