Add unprivileged port validation

[pdabelf5]

Proposed changes

• Add ValidateUnprivilegedPort() to validation package
• Switch to int input to ValidatePort()
• remove port validation function from flags.go

Checklist

Before creating a PR, run through this checklist and mark each as complete.

• I have read the CONTRIBUTING doc
• I have added tests that prove my fix is effective or that my feature works
• I have checked that all unit tests pass after adding my changes
• I have updated necessary documentation
• I have rebased my branch onto main
• I will ensure my PR is targeting the main branch and pulling from my branch from my own fork

[codecov]

Codecov Report

Attention: Patch coverage is 52.38095% with 10 lines in your changes missing coverage. Please review.

Project coverage is 52.76%. Comparing base (becafe7) to head (3d6356f).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
cmd/nginx-ingress/flags.go	0.00%	4 Missing ⚠️
internal/validation/validation.go	75.00%	2 Missing and 1 partial ⚠️
pkg/apis/dos/validation/dos.go	40.00%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7034      +/-   ##
==========================================
+ Coverage   52.73%   52.76%   +0.03%     
==========================================
  Files          89       89              
  Lines       20824    20831       +7     
==========================================
+ Hits        10981    10992      +11     
+ Misses       9389     9382       -7     
- Partials      454      457       +3     

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