chore(admission): adapt admission server to controller-runtime

[czeslavo]

What this PR does / why we need it:

Which issue this PR fixes:

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

Attention: 450 lines in your changes are missing coverage. Please review.

Comparison is base (2a87354) 77.9% compared to head (4366bf8) 66.3%.
Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff            @@
##            main   #4763      +/-   ##
========================================
- Coverage   77.9%   66.3%   -11.7%     
========================================
  Files        163     167       +4     
  Lines      18533   18439      -94     
========================================
- Hits       14451   12228    -2223     
- Misses      3275    5506    +2231     
+ Partials     807     705     -102     

Files	Coverage Δ
internal/admission/utils.go	0.0% <ø> (-81.1%)	⬇️
internal/admission/validator.go	0.0% <ø> (-75.2%)	⬇️
internal/manager/run.go	55.6% <0.0%> (ø)
internal/admission/server.go	0.0% <0.0%> (-42.0%)	⬇️
...nternal/admission/kong_cluster_plugin_validator.go	63.1% <63.1%> (ø)
internal/admission/kong_ingress_validator.go	0.0% <0.0%> (ø)
...nternal/admission/kong_consumer_group_validator.go	53.3% <53.3%> (ø)
internal/admission/kong_plugin_validator.go	59.3% <59.3%> (ø)
internal/admission/custom_validator_adapter.go	0.0% <0.0%> (ø)
internal/admission/ingress_validator.go	0.0% <0.0%> (ø)
... and 5 more

... and 47 files with indirect coverage changes

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