U/u should mean week not month

[jtrakk]

As discussed in https://discourse.julialang.org/t/posix-month/58572, Unix, Posix, Python, and R agree that U and u mean week, not month. In Julia's Dates they mean month. I think Julia should use the common abbreviations here. I'm not sure how to handle backward compatibility though.

[mgkuhn]

Unfortunate, but not strictly a bug, as there is no claim for compatibility with strftime. Other format characters also have ended up with different meanings, e.g. e. There is a more severe and closely-related problem with Dates.DateFormat, namely that the list of 14 format characters is currently fixed (i.e., all other characters are documented to just stand for themselves), see #38959, and we therefore cannot add new format characters without breaking backwards compatibility, so we can't easily add e.g. V for the ISO 8601 week number, etc.

I suspect the entire Dates.DateFormat syntax ought to be superseded at some point in the future with something more flexible and extensible, and that probably should follow much more closely strftime’s % syntax, i.e. each format character should be preceded by an escape character, rather than the other way round.