Consider making Date and DateTime not promote

[LilithHafner]

Make this throw:

julia> using Dates

julia> promote(today(), now())
(DateTime("2023-10-17T00:00:00"), DateTime("2023-10-17T15:47:08.549"))

1, when placed on the floating point number line, unambiguously lands on 1.0
Date("2023-10-17"), when placed on the* continuous timeline, represents a range, not a point.

Recommended for consideration by triage a while ago

*assuming nonrelativistic speeds

[jariji]

Related, incomplete date/time strings parse: #28090

julia> Date("2001-01", dateformat"yyyy-mm-dd")
2001-01-01

julia> DateTime("2001-01", dateformat"yyyy-mm-dd")
2001-01-01T00:00:00