fix: fix webhook validation of KongConsumers referring Secrets with konghq.com/credential label

[pmalek]

What this PR does / why we need it:

After introducing credential type labels in #4825, it seems that webhook wasn't checked for letting the Secrets with only said label (where it should).

This PR fixes it by fixing the webhook logic in KongHTTPValidator.ValidateCredential.

Which issue this PR fixes:

Fixes: #5069

PR Readiness Checklist:

Complete these before marking the PR as ready to review:

No CHANGELOG entry since the change regarding keyCredType wasn't released yet.

- [ ] 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: 5 lines in your changes are missing coverage. Please review.

Comparison is base (c39e31e) 75.5% compared to head (4631b6f) 75.5%.
Report is 1 commits behind head on main.

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #5067   +/-   ##
=====================================
  Coverage   75.5%   75.5%           
=====================================
  Files        167     167           
  Lines      18913   18909    -4     
=====================================
+ Hits       14285   14290    +5     
+ Misses      3798    3793    -5     
+ Partials     830     826    -4     

Files	Coverage Δ
...ion/validation/consumers/credentials/validation.go	92.2% <16.6%> (-4.1%)	⬇️

... and 5 files with indirect coverage changes

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[rainest]

Cross-resource checks, ever a blessing.