[Silabs] dependency inversion on platform depending on app/server/Server.h

[andy31415]

We have:

src/platform/silabs/efr32/wifi/wfx_notify.cpp
44:#include <app/server/Server.h>
                                                                                                                                                                                                                                                                               
src/platform/silabs/SiWx917/wifi/wfx_notify.cpp
37:#include <app/server/Server.h>

It does not seem right that a platform file (low level) depends on app (highest level). This breaks layering and dependencies.
Found while checking #30596

[andy31415]

We also seem to #include <app/server/Dnssd.h> which is not ok.

Also having #include "AppConfig.h" is unclear to me since I am not sure where AppConfig.h resides and what generates it.