Archive knative-extensions/net-certmanager

[ReToCode]

knative-extensions/net-certmanager was integrated into the serving code base, so we can archive the specific repository. The following usages have to be updated:

• https://github.com/search?q=org%3Aknative%20net-certmanager&type=code
• https://github.com/search?q=org%3Aknative-extensions+net-certmanager+NOT+repo%3Aknative-extensions%2Fnet-certmanager&type=code

/assign

[dprotaso]

From: #1564 (comment)

Hey - we technically shouldn't archive net-certmanager until v1.14 and v1.13 has EOL. Which is Oct (ref)

This is in case we neet to cut a point release