Make postgres_version action input default to a string

This is "required" by GitHub Actions, though they must do some coersion
on their side.

.github/actions/neon-project-create/action.yml

@@ -10,7 +10,7 @@ inputs:
     default: aws-us-east-2
   postgres_version:
     description: 'Postgres version; default is 15'
-    default: 15
+    default: '15'
   api_host:
     description: 'Neon API host'
     default: console-stage.neon.build