Fix some warnings in test

[vandalt]

This fixes two warnings I saw when running tests:

1. Deprecation warning for ax.plot_date: fixed by using ax.plot with datetime64 instead
2. Warnings about non-rotation transforms when calculating Sun and Moon separations. Fixed by adding origin_mismach="ignore" to those two calculations. There is already a comment about this before both instances and removing the warning avoids end users seeing it and wondering if something is wrong with how they specified coordinates.

Fix #603

[pllim]

separation() got an unexpected keyword argument 'origin_mismatch'

The problem with this is that astroplan pins to too old of minversion of astropy etc. So we either have to bump minversion or do a version check.

Would love to see this clean up go in!

Also would have to un-ignore the relevant warnings here:

astroplan/setup.cfg

Lines 54 to 57 in bf5e26d

	# Can remove ignore when you bump astropy minversion high enough
	ignore:distutils Version classes are deprecated:DeprecationWarning
	# deprecated in Matplotlib 3.9 and will be removed in 3.11. Use plot instead.
	ignore:The plot_date function was deprecated

Thanks!