Make it an error to try to use the server GC in macOS

[rolfbjarne]

Customer's should have to run into random issues due to this:

https://github.com/xamarin/xamarin-macios/blob/main/runtime/coreclr-bridge.m#L203-L210

we should just make this a build-time error.

It's probably enough to check for the ServerGarbageCollection property:

dotnet/runtime#78280 (comment)

[AustinWise]

There is also the GC.GetConfigurationVariables() API. It returns a dictionary with a key ServerGC that can be checked. This API was added in .NET 7.

[jkotas]

GCSettings.IsServerGC is more straightforward to test for server GC at runtime (this API existed since forever).

[slluis]

@rolfbjarne is there a possibility of adding Server GC support and short/mid term?

[rolfbjarne]

@slluis it could probably be implemented, is it something that would be interesting for VSMac? It's not trivial though, because this is a rather complex area of the runtime.

[slluis]

@rolfbjarne we are having many problems with GC pauses in VS Mac, and I'm being told by @sharwell that enabling Server GC together with other GC options made a huge difference on Windows.