fix x-ms-mutability (#25825)

Author: xiaoxuqi-ms

specification/mysql/resource-manager/Microsoft.DBforMySQL/FlexibleServers/preview/2023-06-01-preview/FlexibleServers.json

@@ -1129,11 +1129,7 @@
             "$ref": "../../../../../../common-types/resource-management/v4/privatelinks.json#/definitions/PrivateEndpointConnection"
           },
           "description": "PrivateEndpointConnections related properties of a server.",
-          "x-ms-mutability": [
-            "create",
-            "update",
-            "read"
-          ]
+          "readOnly": true
         },
         "maintenanceWindow": {
           "$ref": "#/definitions/MaintenanceWindow",