Deprecate constants context manager

As stated in the diff of #8517:

The context manager may be used at any time in a Python session, but it
uses the prior version only for `astropy.constants`, and not for any
other subpackage such as `astropy.units`.

due to this warning (removed in the aforementioned PR):

Units such as ``u.M_sun`` will use the current version of the
corresponding constant. When using prior versions of the constants,
quantities should be constructed with constants instead of units.

With ScienceState enabled for constants and units, perhaps the constants context manager is more confusing than helpful now? Should we deprecate it and ask users to use ScienceState exclusively? Or is there still a need for context manager?

cc @stargaser and @mhvk

[bsipocz]

And since we are not supposed to deprecate stuff in LTS (if I remember @bsipocz correctly), this will have to wait till 4.1?

imo LTS is not the best place to add a new experimental API as then we need to stick with it for quite a while. but LTS is the best place to clean up APIs, deprecate and remove stuff, and to clear up previous deprecations.

[mhvk]

My addendum to APE 2 - https://github.com/astropy/astropy-APEs/pull/20 - was exactly the reverse: that LTS is the right time to add deprecation warnings, since most people will not be supporting two LTS at the same time.