fix: keep @semantic-release/github plugin for GitHub Releases

This plugin creates proper GitHub Release pages with changelogs,
which is valuable independently of workflow triggering.

.releaserc

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