Refactor date handling in wp_insert_post()

[redsweater]

I started refactoring this code while researching #35874, because I thought the unpredictable logic in this function might have contributed to the problems described there. Although that turned out not to be the case, I think the refactoring I've done is still valuable. Effectively I've aimed to clarify the code by leaning heavily on standardizing the validation of submitted dates into the $post_date variable, and then converting that back to a GMT based $post_date_gmt only when all other shared validation is completed.

In my opinion this makes the code easier to read and maintain, but as this doesn't specifically address any known bug, I will appreciate it if it doesn't seem worth integrating.

[redsweater]

Refactor wp_insert_post()'s handling of dates to be more readable and maintainable