renovatebot: Enable rust manager, but only with dashboard approval

see https://docs.renovatebot.com/configuration-options/#dependencydashboardapproval

Author: Turbo87

Diff for: .github/renovate.json

@@ -18,7 +18,7 @@
         "reviewersSampleSize": 1
     },
     "rust": {
-        "enabled": false
+        "dependencyDashboardApproval": true
     },
     "postUpdateOptions": ["yarnDedupeFewer"],
     "packageRules": [{