MNT: Deprecate update_default_config and ConfigurationMissingWarning

[pllim]

This is a follow up of #10877 by @saimn . In the astropy.config subpackage, update_default_config and ConfigurationMissingWarning are no longer used within this package, but some downstream packages might still use them. There is a need to:

• Communicate this to downstream package maintainers, so they have a plan to stop using them too. Memo sent on 2021-04-08 to https://groups.google.com/g/astropy-dev/c/KVzPgeihxO8
• Update package-template to stop using it. See MNT: Stop writing package.cfg on import package-template#509
• Deprecate these two things.

astropy/astropy/config/configuration.py

Lines 43 to 45 in fa5ca6d

	# This was raised with Astropy < 4.3 when the configuration file was not found.
	# It is kept for compatibility and should be removed at some point.
	class ConfigurationMissingWarning(AstropyWarning):

astropy/astropy/config/configuration.py

Lines 713 to 717 in fa5ca6d

	# This function is no more used by astropy but it is kept for the other
	# packages that may use it (e.g. astroquery). It should be removed at some
	# point.
	# this is not in __all__ because it's not intended that a user uses it
	def update_default_config(pkg, default_cfg_dir_or_fn, version=None, rootname='astropy'):

After this is done, open a new issue to remove them altogether from the codebase after a few more releases.