fix: add @semantic-release/github plugin to create GitHub Releases

The update-major-tag workflow triggers on `release: published`, but
semantic-release was only creating git tags (via @semantic-release/git)
without creating actual GitHub Release objects. This meant the workflow
never fired.

.releaserc

@@ -17,6 +17,7 @@
       {
         "assets": ["docs/CHANGELOG.md", "package.json"]
       }
-    ]
+    ],
+    "@semantic-release/github"
   ]
 }