Fix accidental non-collectible context unloading

Fix accidental non-collectible context unloading

The OnProcessExit was iterating over the s_contextsToUnload and calling
Unload on each of the contexts. However the s_contextsToUnload is a
misnomer, it contains all the contexts, both collectible and
non-collectible ones. So we were calling Unload on non-collectible
contexts too there. That resulted in InvalidOperationException being
thrown from the OnProcessExit.

This change fixes that by calling unload only on ALCs whose
IsCollectible returns true. It also renames the s_contextsToUnload to
s_allContexts.

Close #22902