Remove deprecated packages & staticcheck fixes (#22012)

`ioutil` is deprecated and should use `io` instead

modules/activitypub/client_test.go

@@ -35,7 +35,7 @@ func TestActivityPubSignedPost(t *testing.T) {
 		body, err := io.ReadAll(r.Body)
 		assert.NoError(t, err)
 		assert.Equal(t, expected, string(body))
-		fmt.Fprintf(w, expected)
+		fmt.Fprint(w, expected)
 	}))
 	defer srv.Close()