Fix thread pool hang (#56346)

Fix thread pool hang (#56346)

* Fix thread pool hang

- In https://github.com/dotnet/runtime/pull/53471 the thread count goal was moved out of `ThreadCounts`, it turns out that are a few subtle races that it was avoiding. There are other ways to fix it, but I've added the goal back into `ThreadCounts` for now.
- Reverted PR https://github.com/dotnet/runtime/pull/55985, which worked around the issue in the CI

Fixes https://github.com/dotnet/runtime/issues/55642

* Revert "mitigation for quic tests hangs (#55985)"

This reverts commit 0a5e93b09fe92cf866456552eef78a95bf6fdf27.