docs: upgrade note for ingress.class on KCP

[shaneutt]

What this PR does / why we need it:

KongClusterPlugins used to be usable without having the ingress.class
explicitly set like other resources (such as Ingress) require. As we've
seen some instances in the community where there's been confusion
about this, this patch retroactively adds this as a breaking change note
to the initial 2.0.x release in hopes that this will help reduce confusion
when upgrading.

Which issue this PR fixes

Fixes #2090

PR Readiness Checklist:

• the CHANGELOG.md release notes have been updated

[mflendrich]

KongClusterPlugins used to be usable without having the ingress.class explicitly set like other resources (such as Ingress) require.

seems to contradict the docs for v1.x (see the comment).

Let's clarify what the actual breaking change here is, because maybe there's none?