Ability to remove variable from using experssion

[HakamFostok]

This code compiles

using (StartObservabilityActivity())
{
	// everything is intendent
}

but this code does NOT compile

using StartObservabilityActivity();
// remaining of the scope

I am suggesting to allow the second code

Motivation

the motivation behind this is: that since .NET 6, the observability started to take momentum and now there is a lot of creating Activity which only need to be created and disposed, but nothing else (in most cases).
This will prevent the unnecessary indentation of the whole code that need to be observed.
Otherwise, we have to set it to the _ variable to be able to use the using expression.

[HaloFour]

See: #2235