Diff router is not compatible with config-sync

Diff is quite different from other routers and requires that it has exactly two servers as targets. Further, one of those targets must be the argument of the setting main. That can be enforced when Diff is created at runtime and also when Diff is created from a configuration file.

However, when configuration synchronization is used, the Diff router will be created in a somewhat different manner, which leads to those checks always failing. Currently it is not possible to work around this, so for 25.01 it is documented that Diff is not compatible with configuration synchronization and it is also checked when a Diff service is created whether configuration synchronization is enabled, and if it is, the creation will fail.