Improve error message when targeting net6.0-ios and net6.0-macos from net 8

[haritha-mohan]

Recently when trying to fix our sample submissions in maccore using the net8.0-xcode15 branch of macios, I was running into the following issue for the dotnet apps:

error NETSDK1147: To build this project, the following workloads must be installed: wasm-tools-net6
error NETSDK1147: To install these workloads, run the following command: dotnet workload restore

..which doesn't make much sense why wasm is needed for these projects. Turns out this is the error displayed when targeting net6 from net8. The android SDK team faced a similar issue and addressed it and I think we can follow suit.

we dropped net6 support for net8 here: #17901
XA issue: dotnet/android#8003
XA fix: dotnet/android#8047

Wondering if we could do the same? I can implement the fix, just want to check if this is the correct course of action.

[rolfbjarne]

I'm already working on something like this (as part of a fix for dotnet/sdk#30103), so I can do this.

[rolfbjarne]

Fixed in #18931.