[DX] Add the ability to deprecate the name (and value?) of config entries

This is a sibling issue to #5740. The same idea, but for entries in config files.

I envision the ability (via hooks?) to allow core and contrib modules/themes to specify names of deprecated config names/keys, and then map those to the respective new ones. If there is need to also manipulate the values for these keys, then perhaps also allow specifying functions that do that.

The idea is that if code calls config_get()/Config::get or config_set()/Config::set with a deprecated key name, then we automatically "redirect" these calls to read/save the new, replacement keys instead. We could also be logging "deprecated config entry called" messages, so that contrib can notice these and fix their code to not use the deprecated config key names in the first place.

This would help us in issues like #4569, where we have the need to rename a config key, but because that is also used in contrib-provided configuration files, we'd have to result in retaining the old name for backwards-compatibility reasons, even if the name and respective values would not make sense.