lib: Always use ISO format for date picker manual input

[martinpitt]

PatternFly's DatePicker's textual date input defaults to ISO format
(YYYY-MM-DD), for a good reason: it's unambiguous, and localized date
input is really hard and brittle.

This is also broken for e.g. en-UK: British date format is DD/MM/YY,
while US date format is MM/DD/YY, but we don't have UI to switch between
English countries.

It was also broken for the manual time setting dialog, where the initial
formatting of the date resulted in a format that the dialog didn't
accept.

So give up, and use the default (i.e. ISO) format and parser. Also drop
the custom time/date format function in serverTime.js, and use our
standard timeformat.dateTime() (which has the same output format).

Fixes #19091

[1] https://www.patternfly.org/components/date-and-time/date-picker

---

It previously looked like this in English (scheduled shutdown time):

And now like this, in all languages (it's not an off-by-one, I changed the day to tomorrow):

When emptying the input you see the corresponding input helper:

And for the LOLz, the buggy default dialog value for manually setting the server date on main. With this PR, it's the same ISO format as above:

I suppose this only didn't get a bug report because nobody actually does that..

When people say "hard times" I didn't realize what they mean.. 😅

[martinpitt]

Matrix conversation:

17:16:50      pitti │ Garrett: quick gut check: you remember our calendar selector widget, in the timer create and scheduled shutdown dialogs? they also offer you to manually type in the date                                             
17:17:39      pitti │ Garrett: how would you feel like if we stop trying to interpret a locale specific date (what's 03/08/2024? Is it UK or US English?) and just supported ISO format? yyyy-mm-dd; it should be ok for our target audience
17:18:06      pitti │ also, we don't even have an option to select British locale, so it's even broken for them right now (switching month and day)                                                                                         
17:21:12    Garrett │ yeah, sure                                                                                                                                                                                                            
17:21:15    Garrett │ that makes sense                                                                                                                                                                                                      
17:21:32    Garrett │ yeah, we don't have a british or intl. english                                                                                                                                                                        
17:21:38    Garrett │ and that would remove confusion too                                                                                                                                                                                   
17:21:55    Garrett │ and, you're right that our audience is probably familiar with yyyy-mm-dd                                                                                                                                              
17:23:37      pitti │ we've also fixed this part like three times for different languages, it's so brittle                                                                                                                                  
17:23:52    Garrett │ yeah                                                                                                                                                                                                                  
17:23:58      pitti │ Garrett: I'm asking because there's no real browser API for localized date input/parsing, so this is annoyingly hard                                                                                                  

wrt. this PR:

Garrett │ pitti: looks mostly fine. Invalid date format warning should be under the date, not time.

This isn't just a bug, more of a mis-design. If both are invalid, we show it like this:

Presumably to avoid the strings becoming too long and not fitting next to each other?

So let's do this as a follow-up, see issue #20669

[jelly]

LGTM

[allisonkarlitskaya]

Nice cleanup, thanks!