Improvements to subscribing to events

[manodasanW]

• Remove calls to Mono.ThreadContext on .NET 5 as it is for .NET standard 2 support
• Move the manual projection for IWeakReference to use the new updated format that provides an alternative to IDIC
• Remove the vftbl for IWeakReference as it is not a generic type
• Take advantage of both of the above in event sources to get a weak reference
• Move both the Cache and state class outside of event source so that there isn't a version of it per delegate type and so that we are not creating a new set of dictionaries (both the static one and the instance one) per delegate type.
• Reduce the cost of getting a weak reference by reducing allocations.

[Scottj1s]

looks good!