chore: deprecate KongIngress entirely

[czeslavo]

What this PR does / why we need it:

Makes .Service and .Route fields not allowed with the use of the CRD validations. Makes .Upstream deprecated by returning a warning from the admission webhook and not using the field at all.

Which issue this PR fixes:

Closes #4720.

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 and coverable lines are covered by tests ✅

Comparison is base (8c727d9) 75.3% compared to head (80f2198) 75.5%.
Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #5015     +/-   ##
=======================================
+ Coverage   75.3%   75.5%   +0.2%     
=======================================
  Files        167     167             
  Lines      18719   18526    -193     
=======================================
- Hits       14097   13993    -104     
+ Misses      3794    3721     -73     
+ Partials     828     812     -16     

Files	Coverage Δ
internal/adminapi/backoff_strategy_konnect.go	100.0% <ø> (ø)
internal/adminapi/client.go	55.8% <ø> (ø)
internal/adminapi/endpoints.go	84.2% <ø> (ø)
internal/adminapi/kong.go	61.9% <ø> (ø)
internal/adminapi/konnect.go	0.0% <ø> (ø)
internal/admission/adminapi_provider.go	89.4% <ø> (ø)
internal/admission/handler.go	58.6% <100.0%> (-1.0%)	⬇️
internal/admission/utils.go	81.0% <ø> (ø)
...ion/validation/consumers/credentials/validation.go	96.2% <ø> (ø)
internal/admission/validation/gateway/httproute.go	90.0% <ø> (ø)
... and 110 more

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[czeslavo]

Closing in favor of #5022.