fix: update major version tag as post-release step

Move major tag update (e.g. v6) into the release workflow as a
conditional step, gated on semantic-release actually publishing.
This avoids the GITHUB_TOKEN limitation where events from the
default token don't trigger separate workflows.

Also reverts @semantic-release/github from .releaserc since we no
longer rely on the release event to trigger a separate workflow.

.releaserc

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