Update Yarn to v4.9.4 (#35837)

Co-authored-by: renovate[bot] <29139614+renovate[bot]@users.noreply.github.com>

package.json

@@ -1,7 +1,7 @@
 {
   "name": "@mastodon/mastodon",
   "license": "AGPL-3.0-or-later",
-  "packageManager": "yarn@4.9.2",
+  "packageManager": "yarn@4.9.4",
   "engines": {
     "node": ">=20"
   },

streaming/package.json

@@ -1,7 +1,7 @@
 {
   "name": "@mastodon/streaming",
   "license": "AGPL-3.0-or-later",
-  "packageManager": "yarn@4.9.2",
+  "packageManager": "yarn@4.9.4",
   "engines": {
     "node": ">=20"
   },