chore: remove deprecated KongIngress functionality

[czeslavo]

What this PR does / why we need it:

Stops respecting deprecated KongIngress Proxy and Route fields and aligns tests to use annotations in favor of KongIngress.

Which issue this PR fixes:

Closes #3018 and #4740.

Special notes for your reviewer:

PR Readiness Checklist:

Complete these before marking the PR as ready to review:

• the CHANGELOG.md release notes have been updated to reflect any significant (and particularly user-facing) changes introduced by this PR

[codecov]

Codecov Report

All modified lines are covered by tests ✅

Comparison is base (edfd7e2) 67.5% compared to head (76981cd) 77.7%.
Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff            @@
##            main   #4760      +/-   ##
========================================
+ Coverage   67.5%   77.7%   +10.1%     
========================================
  Files        163     163              
  Lines      18667   18526     -141     
========================================
+ Hits       12609   14403    +1794     
+ Misses      5323    3315    -2008     
- Partials     735     808      +73     

Files	Coverage Δ
internal/admission/handler.go	61.2% <100.0%> (+28.4%)	⬆️
internal/dataplane/kongstate/kongstate.go	79.1% <100.0%> (+3.8%)	⬆️
internal/dataplane/kongstate/route.go	97.5% <100.0%> (+6.5%)	⬆️
internal/dataplane/kongstate/service.go	68.4% <100.0%> (+21.6%)	⬆️
internal/dataplane/kongstate/util.go	89.0% <ø> (-0.3%)	⬇️

... and 49 files with indirect coverage changes

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