Consistent BitEnum Naming

[Le0Developer]

Summary

Currently not all bit-enums are created equal.
The following flags should be pluralized:

• ActivityFlag
• ChannelFlag
• GuildSystemChannelFlag
• MessageFlag
• UserFlag

Why is this needed?

Consistency.

Ideal implementation

Add an s.

Checklist

• I have searched the issue tracker and have made sure it's not a duplicate. If it is a follow up of another issue, I have specified it.

[Le0Developer]

Deprecation will be a bit tricky but can be done with a module level __getattr__.

Example:

_deprecated = {"ActivityFlag": ActivityFlags, ...}
def __getattr__(name: str) -> typing.Any:
    if name in _deprecated:
        do_deprecation_warning()
        return _deprecated[name]
    raise AttributeError(f"module {__name__!r} has no attribute {name!r}")