[std] Update references from ISO 8601:2004 to ISO 8601-1:2019

[jensmaurer]

As requested by ISO/CS for C++23 DIS.

[tkoeppe]

@Dani-Hub Could you please take a look? Jens looked up the new subclause number in the table of contents.

[Dani-Hub]

I wonder whether referring to 5.3.4.1 alone is too limited. I suggested 5.3.4 because it contains both 5.3.4.1 and 5.3.4.2, where the latter additionally explains how to combine the time of day in the presence of a time shift between local time and UTC. But I acknowledge that this part may not be needed for %z.

[jensmaurer]

where the latter additionally explains how to combine the time of day in the presence of a time shift between local time and UTC.

Nowhere do we discuss the combination in C++. %z just has the offset, which is defined in 5.3.4.1.

[tkoeppe]

Great, thank you, both!