Make DE 440 default ephemeris (and other follow-up)

[mhvk]

Following up on #11486:

1. Probably, DE440 should replace DE430 as the default ephemeris. See Make DE440s the default JPL ephemeris #11608 for a PR and possible issues.
2. It would be good to check very occasionally that the abbreviations we introduced are actually able to retrieve a file. Perhaps this can just be a test that the implied link exists rather than an actual download?

[pllim]

Does this need to happen for 5.0? If so, it is one of the rare issues that we should milestone so we don't forget.

[mhvk]

Good point, added the milestone.

[pllim]

I am now disagreeing with my Past Self ™️ and removing the milestone, given that #11608 has one but the second item on the list above has no urgency w.r.t. release.