No Ply Restriction in the condition that limits the depth extension to a certain point

[FauziAkram]

I was inspired by shawn's tests, which tried to remove the condition completely.
Interestingly, the patch passed STC but quickly failed LTC.

Reference:
shawn complete removal STC (Passed):
https://tests.stockfishchess.org/tests/view/67350b3a86d5ee47d953ea8f
shawn complete removal LTC (Failed):
https://tests.stockfishchess.org/tests/view/6736825586d5ee47d953ec97

Now, in this implementation, the depth extension is no longer tied to the current ply within the search tree, it now limits it based on the root depth itself. The extension will only be applied if the initial root search depth is greater than 8.

Passed STC:
https://tests.stockfishchess.org/tests/view/6767f4bb86d5ee47d9544582

Passed LTC:
https://tests.stockfishchess.org/tests/view/67695f9986d5ee47d954499c

bench: 926381

[Disservin]

this is barely a simplification and i'd prefer the dynamic limit over the fixed rootdepth one, we can change this if something passes gainer bounds imo

[xu-shawn]

i think it does make sense tbh. Perhaps the old simplification fails due to explosion at low depth searches with a filled TT, which IMO is better guarded by this clause than the one we currently have.

[vondele]

I'm going to close this for now, since it has gotten pretty outdated. However, can be retested after the next merges IMO.