Remove use of conservative stack scanning mode in CoreCLR

[joshpeterson]

For the time being, Unity requires that the CoreCLR GC uses its conservative stack scanning mode. Longer term, we want to change the way that native code in Unity interacts with managed code so that we no longer need to use this GC mode.

See dotnet#61919 for more discussion in the upstream repo.

[joshpeterson]

Since current CoreCLR upstream code has removed OS-level thread suspension APIs and the CoreCLR GC code is not save to call with threads suspended in native code (e.g. the GC code might call malloc when a native thread is suspended while holding the allocator lock), we have decided not to use the conservative stack scanning mode in CoreCLR.

Instead, we will modify the Unity code to avoid the access to managed memory in native code, so that we can avoid the need to suspend threads in native code and scan native stack frames.