fix(conf) fix creation of non-default IngressClasses

[ONordander]

When using a custom IngressClass we should check for existence of that class, not the default 'kong' one.

What this PR does / why we need it:

If a user wants to have two Kong instances installed with different IngressClasses the second installation will not actually create the IngressClass since it checks for the existence of the 'kong' IngressClass.

Special notes for your reviewer:

Tested with:

ingressController:
  ingressClass: test-kong

and
helm template --validate -f values.yaml kong -n kong .

Checklist

[Place an '[x]' (no spaces) in all applicable fields. Please remove unrelated fields.]

• PR is based off the current tip of the main branch.
• Changes are documented under the "Unreleased" header in CHANGELOG.md
• Commits follow the Kong commit message guidelines

[CLAassistant]

All committers have signed the CLA.

[shaneutt]

This LGTM, would also like a double check from @rainest