Deprecate Knative Backstage plugins

[aliok]

https://github.com/knative-extensions/backstage-plugins

Let's deprecate the components here as there's no interest at all.

cc @knative/steering-committee @knative/eventing-reviewers

[aliok]

I actually don't know if the process for deprecation is written somewhere.

[dprotaso]

@aliok we have some steps here: https://github.com/knative/community/blob/main/mechanics/SUNSETTING-FEATURES.md

In general though I think it depends on the scope and GA level of the component.