(PostgreSQL) fixes Azure/azure-sdk-for-go#18506 (#19661)

Azure is offering postgres 14 flexible server but the sdk is currently limited to Versions 11 12 and 13.

https://github.com/Azure/azure-sdk-for-go/blob/sdk/resourcemanager/postgresql/armpostgresqlflexibleservers/v1.0.0/sdk/resourcemanager/postgresql/armpostgresqlflexibleservers/zz_generated_constants.go#L263

fixes Azure/azure-sdk-for-go#18506

Author: navba-MSFT

specification/postgresql/resource-manager/Microsoft.DBforPostgreSQL/stable/2021-06-01/postgresql.json

@@ -1391,6 +1391,7 @@
       "type": "string",
       "description": "The version of a server.",
       "enum": [
+        "14",
         "13",
         "12",
         "11"