Find degrees of freedom for `non_central_f` distribution #927

Job	Run time
posix (Collect coverage interpolators, yes, gcc-12, 20, ubuntu-22.04, g++-12-multilib, 64, interp...	28m 42s
posix (Collect coverage distribution_tests, yes, gcc-12, 20, ubuntu-22.04, g++-12-multilib, 64, d...	6m 28s
posix (Collect coverage reverse-autodiff, yes, gcc-12, 20, ubuntu-22.04, g++-12-multilib, 64, tes...	5m 1s
posix (Collect coverage quadrature, yes, gcc-12, 20, ubuntu-22.04, g++-12-multilib, 64, quadrature)	11m 49s
posix (Collect coverage special_fun, yes, gcc-12, 20, ubuntu-22.04, g++-12-multilib, 64, special_...	13m 9s
posix (Collect coverage autodiff, yes, gcc-12, 20, ubuntu-22.04, g++-12-multilib, 64, autodiff)	6m 14s
posix (Collect coverage gradient-optimizers, yes, gcc-12, 20, ubuntu-22.04, g++-12-multilib, 64, ...	4m 13s
posix (Collect coverage misc, yes, gcc-12, 20, ubuntu-22.04, g++-12-multilib, 64, misc)	7m 45s
posix (Collect coverage mp, yes, gcc-12, 20, ubuntu-22.04, g++-12-multilib, 64, mp)	4m 7s
	1h 27m 28s

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