[release/8.0] Add boolean field to indicate whether or not the Windows thread pool...

[release/8.0] Add boolean field to indicate whether or not the Windows thread pool is being used (#91246)

* Add boolean field to indicate whether or not the Windows thread pool is being used

* Update src/libraries/System.Private.CoreLib/src/System/Threading/ThreadPool.Windows.cs

Co-authored-by: Stephen Toub <stoub@microsoft.com>
* Remove trailing spaces

* Move comment

* Update fields and comments

* Disable CA1823 - unused field

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Co-authored-by: unknown <eduardo.velarde@pucp.pe>
Co-authored-by: Eduardo Velarde <32459232+eduardo-vp@users.noreply.github.com>
Co-authored-by: Stephen Toub <stoub@microsoft.com>