MAINT: switch to cumulative_trapezoid
I noticed this test warning so I'm going to make the needed changes here.
test/gw/plot_test.py: 18 warnings
/builds/lscsoft/bilby/bilby/core/prior/interpolated.py:169: DeprecationWarning: 'scipy.integrate.cumtrapz' is deprecated in favour of 'scipy.integrate.cumulative_trapezoid' and will be removed in SciPy 1.14.0
self.YY = cumtrapz(self._yy, self.xx, initial=0)