Change references to tootsuite/mastodon to mastodon/mastodon (#16491)

* Change references to tootsuite/mastodon to mastodon/mastodon

* Remove obsolete test fixture

* Replace occurrences of tootsuite/mastodon with mastodon/mastodon in CHANGELOG

And a few other places

package.json

@@ -1,5 +1,5 @@
 {
-  "name": "@tootsuite/mastodon",
+  "name": "@mastodon/mastodon",
   "license": "AGPL-3.0-or-later",
   "engines": {
     "node": ">=12"
@@ -18,7 +18,7 @@
   },
   "repository": {
     "type": "git",
-    "url": "https://github.com/tootsuite/mastodon.git"
+    "url": "https://github.com/mastodon/mastodon.git"
   },
   "browserslist": [
     "last 2 versions",