chore: remove everything related obsolete global KongPlugins

[programmer04]

What this PR does / why we need it:

KongClusterPlugin supports the label global: true, see the docs and it is not changed.

For a long time (since KIC v0.10.0) for KongPlugin with the label global: true KIC logs a warning about unsupporting. This PR gets rid of everything related to the support of label global for KongPlugin, including a warning.

Which issue this PR fixes:

Closes #3873

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 (95b0447) 67.9% compared to head (59e302b) 67.8%.
Report is 1 commits behind head on main.

❗ Current head 59e302b differs from pull request most recent head 90347c6. Consider uploading reports for the commit 90347c6 to get more accurate results

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #4737     +/-   ##
=======================================
- Coverage   67.9%   67.8%   -0.1%     
=======================================
  Files        165     165             
  Lines      19291   19208     -83     
=======================================
- Hits       13102   13029     -73     
+ Misses      5420    5416      -4     
+ Partials     769     763      -6     

Files	Coverage Δ
internal/dataplane/kongstate/kongstate.go	79.9% <ø> (+5.8%)	⬆️
internal/store/store.go	61.7% <ø> (-0.1%)	⬇️

... and 9 files with indirect coverage changes

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[team-k8s-bot]

E2E (targeted) tests with KIND-based clusters were started at https://github.com/Kong/kubernetes-ingress-controller/actions/runs/6338093891