Reference issue

None

What does this implement/fix?

• Use von Mises CDF from xsf (see ENH: Implement CDF of the von Mises distribution xsf#111) and remove old Cython implementation from scipy.stats._stats
• Pin
• the xsf submodule to the newest commit

Additional information

• @dschmitz89 had worked on this as well MAINT: use von Mises CDF from xsf #25178
• This is my first PR moving a C++ function from xsf to SciPy. I hope this is the correct and most natural approach.

AI Generation Disclosure

I used Codex to ask questions for the move. I then updated some files myself trying to follow @steppi's suggestion