fix(renovate): target main instead of nonexistent dev branch

baseBranches pointed at a dev branch that does not exist on the remote,
so Renovate has been silently opening no update PRs at all.

renovate.json

@@ -1,7 +1,7 @@
 {
   "$schema": "https://docs.renovatebot.com/renovate-schema.json",
   "extends": ["config:recommended"],
-  "baseBranches": ["dev"],
+  "baseBranches": ["main"],
   "ignoreDeps": ["dev.flutter.flutter-plugin-loader"],
   "automerge": true,
   "automergeType": "pr",