Support for PostgreSQL Flexible Server version 15 (#24203)

* Update FlexibleServers.json

PostgreSQL version 15 is now available in public preview

* Update FlexibleServers.json

* Update FlexibleServers.json

Author: navba-MSFT

specification/postgresql/resource-manager/Microsoft.DBforPostgreSQL/preview/2023-03-01-preview/FlexibleServers.json

@@ -376,6 +376,7 @@
       "type": "string",
       "description": "The version of a server.",
       "enum": [
+        "15",
         "14",
         "13",
         "12",