Re-evaluate Copy Task explicit Thread management for Multithreaded scenarios

### Summary

The Copy Task today manages a static pool of threads to do copies with (see [here](https://github.com/dotnet/msbuild/blob/main/src/Tasks/Copy.cs#L54-L74)). This could lead to contention in MT scenarios.

### Background and Motivation

See aboveIn multiprocess mode, this results in each worker node that runs the Copy task having a dedicated 4- or 6-thread pool for those copies.

In multithreaded mode, that results in one 4- or 6-thread pool being used for all Copy task invocations, since they now all happen on Thread-backed worker nodes, not separate processes.

### Proposed Feature

We should evaluate changing this strategy for multithreaded mode - especially if testing and measurement shows that the existing shared pool is constantly working.

### Alternative Designs

_No response_