[wasm-mt] Revisit the size of pthread pool for CI

[simonrozsival]

Current values are:
pthreadPoolInitialSize = 7
pthreadPoolUnusedSize = 3;

which allows even some very natry threading unit tests to pass.
But WebWorker is expensive resource, especially on Mobile device.
What would be minimum which still works ?

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Issue Details

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In #75286 we increased the size of the pthread pool from the default 4 to 8. This was necessary to make the System.Threading.Tasks.Dataflow.Tests pass. We should revisit this change and determine if it is really necessary and what would be necessary to do to use the default pool size again.

Author:	simonrozsival
Assignees:	-
Labels:	arch-wasm, area-Build-mono
Milestone:	-

[pavelsavara]

navigator.hardwareConcurrency

[pavelsavara]

How many threads we need:

• portable timer
• finalizer
• there could be GC workers in sgen, I'm not sure about it
• thread pool, I think there is one "gate" thread and maybe 2 workers
• we may have debugger thread ?
• we may have event pipe thread ?
• we may have sampling profiler ?