Archive eventing-contrib repo

[vaikas]

As per:
https://github.com/knative/community/blob/main/REPOSITORY-GUIDELINES.md

As you all are probably aware, we had moved most of the code out of eventing-contrib a long time ago.Last release we had was in Dec 15th, 2020https://github.com/knative/eventing-contrib/releases

Left behind were few tools that we use in docs and examples. They have been moved to eventing here:knative/eventing#5295

Therefore, I'd like to finalize the cleanup and archive the repo.
@grantr @rhuss @markusthoemmes @evankanderson

Please let us know if you have any concerns or if it's fine to proceed.

[evankanderson]

+1 from me

[markusthoemmes]

LGTM

[vaikas]

Unless I hear otherwise in the next day, I'll start on this tomorrow then. Thank you!

[vaikas]

Alrighty. I'll work on this list:

• The repo description is edited to start with the phrase "[EOL]"
• All open issues and PRs are closed
• All external collaborates are removed
• All webhooks, apps, integrations, or services are removed
• GitHub pages are disabled
• The repo is marked as archived using GitHub's archive feature
• The removal is announced on the knative-dev mailing list