This PR prevents updating isCustom on object and field metadata.
Example graphql mutation now cannot be used to modify existing field's
isCustom property as expected.
```
mutation UpdateOneFieldMetadataItem(
$idToUpdate: UUID!
$updatePayload: UpdateFieldInput!
) {
updateOneField(input: { id: $idToUpdate, update: $updatePayload }) {
id
isCustom
}
}
```
Example payload is not allowed now and it will return error in response.
```
{
"idToUpdate": "80c6c8df-f719-42d2-985a-353468a3ed00",
"updatePayload": {
"isCustom": true
}
}
```
fixes- #6581
253a3eb83f