gh-103596: [Enum] do not shadow mixed-in methods/attributes (GH-103600)

For example:

    class Book(StrEnum):
        title = auto()
        author = auto()
        desc = auto()

    Book.author.desc is Book.desc

but

    Book.author.title() == 'Author'

is commonly expected.  Using upper-case member names avoids this confusion and possible performance impacts.

Co-authored-by: samypr100 <3933065+samypr100@users.noreply.github.com>