Mark unused&immature activitypub as "not implemented" (#36789)

After many years, "activitypub" is still "in progress" and no real
progress for end users. So it is not mature.

Temporarily mark the endpoints as "501 not implemented",
and wait until the whole design is stable and usable.

.golangci.yml

@@ -51,8 +51,6 @@ linters:
               desc: do not use the go-chi cache package, use gitea's cache system
             - pkg: github.com/pkg/errors
               desc: use builtin errors package instead
-            - pkg: github.com/go-ap/errors
-              desc: use builtin errors package instead
     nolintlint:
       allow-unused: false
       require-explanation: true