gh-91456: [Enum] Deprecate default auto() behavior with mixed value types (GH-91457)

When used with plain Enum, auto() returns the last numeric value assigned, skipping any incompatible member values (such as strings); starting in 3.13 the default auto() for plain Enums will require all the values to be of compatible types, and will return a new value that is 1 higher than any existing value.

Co-authored-by: Ethan Furman <ethan@stoneleaf.us>