ci: configure semrel branches for 2.1 prerelease packages

Author: CharlieC3

package.json

@@ -63,7 +63,12 @@
   },
   "engineStrict": true,
   "release": {
-    "plugins": [
+      "branches": [
+        "master",
+        {"name": "beta", "channel": "beta", "prerelease": true},
+        {"name": "stacks-2.1", "channel": "stacks-2.1", "prerelease": true}
+      ],
+        "plugins": [
       "@semantic-release/commit-analyzer",
       "@semantic-release/release-notes-generator",
       [