Make PolicyBuilder.extension_policies keyword-only.

[deivse]

#12432 (comment)

I decided to remove the new_ prefix from the argument names, because that is a bit too annoying to write out for me. There is a slight inconsistency now with how the other arguments are named, but I can personally live with that.
If we do want to keep it consistent, I would rather remove new_ from all PolicyBuilder argument names than force users to write out new_ee_policy and new_ca_policy every time 😅 But that's just my personal preference of course 😄

[deivse]

Regarding the naming, I just searched through the code quickly, and I don't think any other builder APIs use the new_xxx naming scheme, so I guess removing the new_ prefix from all parameter names would actually improve overall consistency. I'm going to do that preliminarily, and I can revert if you don't agree.