ci: use node v20 for ci (#836)

Author: RebeccaStevens

.github/actions/prepare/action.yml

@@ -5,7 +5,7 @@ inputs:
   node-version:
     required: false
     description: "`node-version` passed to `actions/setup-node`."
-    default: latest
+    default: "20"
 
 runs:
   using: "composite"