[Mono.Parallel] Removed broken Mono.Threading.Tasks code

The code in MonoTaskExtensions.cs relied on the internal method Execute(Action<Task>) in System.Threading.Tasks.Task which was removed in 0a66e60.
Removed NET_2_1 conditional blocks from FileInfo. Fixes #19862.
[amd64] Simplify mono_arch_handle_altstack_exception () by passing a MonoContext on the stack instead of an os specific sigctx structure which required special handling and broke when _XOPEN_SOURCE was defined.
Merge pull request #1067 from esdrubal/bug19862

Removed NET_2_1 conditional blocks from FileInfo. Fixes #19862.
Add System.ServiceModel.Security assembly. Fixes #20108
[verifier] Fix constrained type unwrap for enum types. Fixes #18371
[mcs] Use constrained prefix for enum calls too
Fixes build with newer mcs
[mcs] If the overriding property is sealed, then the overridden accessors cannot be inaccessible
[verifier] Set correctly stack type based on constrained type
Test that ConcurrentQueue does not keep old references. Covers 18421.
Overwrites dequeued ref/value with default. Fixes 18421.
ConcurrentQueue was keeping the last dequeued ref/value.
[sdb] Avoid clearning event requests if they reference an assembly which is unloaded. Instead, remove the assembly reference from the event request so it stays active. Fixes #19924.
[mobile] Add support for Environment.SetEnvironmentVariable in the mobile profile (e.g. iOS)
Bump 'master' branch version.
[jit] Applied patch from Vladimir Kargov <kargov@gmail.com>. Fix a local register allocation problem involving global registers and x86 register constraints.
[gc] Add finalizer API, implemented only on sgen.

The new finalizer API allows an embedder to be notified during a collector
when an object is queued for finalization.

This is a VERY low level interface, as this is called with the world stopped, so the
code MUST be async safe. If you don't precisely know what that means, don't use it.
[boehm] Implement the finalization extension API.
Test ConcurrentDictionary.Contains(KeyValuePair<TKey, TValue>). Covers #16990.
Changed ConcurrentDictionary.Contains(KeyValuePair<TKey, TValue>) to also compare value. Fixes #16990.