Switch macOS and Linux default toolchain to .NET 5.

[pchote]

This PR should cover all the remaining details to switch our default toolchain to .NET 5, keeping compatibility with mono for those who want it on macOS and Linux.

[pchote]

The updated install document can be viewed in a readable form at https://github.com/pchote/OpenRA/blob/net5-makefile/INSTALL.md

[pchote]

We should also merge #19006 first to avoid unnecessary rebase pain wrt the prep branch.

[pchote]

Updated to include the configure-system-libraries.sh fix from #19057.

The build refuses to run with Illegal instruction: 4 after compiling using TARGETPLATFORM=unix-generic, so this will require further investigation.

[teinarss]

Updated to include the configure-system-libraries.sh fix from #19057.

The build refuses to run with Illegal instruction: 4 after compiling using TARGETPLATFORM=unix-generic, so this will require further investigation.

On mono or dotnet, both?

[pchote]

Only on dotnet, only for libsdl2.dylib, and only on macOS. It works fine on Linux (provided the system has a recent enough version of SDL2 installed), and works fine on macOS for the other dylibs. When I tried compiling my own version of SDL2 locally, the game freezes during startup under both mono and dotnet.

This was working fine when I first opened the PR, so I don't understand what has broken...

Since unix-generic is still working as expected under linux, and macOS users don't generally expect this kind of feature to be available, my solution is to remove mention of unix-generic under the macOS build docs.

[pchote]

Rebased now that #19006 has been merged.

[pchote]

Updated.

[Mailaender]

Changelog