Merge pull request #1924 from porter-dev/belanger/add-postgres-13.6

Add postgres `13.6` to RDS minor versions

api/server/handlers/infra/forms.go

@@ -273,6 +273,8 @@ tabs:
           value: "13.3"
         - label: "v13.4"
           value: "13.4"
+        - label: "v13.6"
+          value: "13.6"
   - name: additional-settings
     contents:
     - type: heading