Improve `LightningEnum` and its deprecation

[ethanfurman]

Proposed refactor

• Use the built-in Enum capabilities to make LightningEnum simpler.
• Modify and rename the _OnAccessEnumMeta type to only call a deprecate() method (it's not used for anything else.
• Create a DeprecatedEnum base class and use that instead of constantly setting metaclass=... on deprecated enums.
• Correct attribution for _OnAccessEnumMeta (current attribution is to a website and article that falsely claims authorship of Enum).

Motivation

Fixing the attribution (which came to my attention due to an aenum issue, and efficient use of Enum.

cc @justusschock @awaelchli @rohitgr7 @tchaton

[awaelchli]

@ethanfurman Is this fixing a recursion error you mention?
Would it be better to mark this as a bugfix and include it in 1.6.x?
cc @carmocca