Bump WordPress and PHP minimums

[vikrampm1]

Bump WP & PHP Minimums to the following

• [ ] WordPress to 5.7
• [ ] PHP to 7.4
• [ ] Remove any no-longer-needed conditional code for older version support.
• [ ] Take a look at adjusting our testing matrices, if applicable.
• [ ] Bump the minimums in plugin documentation and header fields.

Misc

• [ ] gutenberg_get_jed_locale_data -- remove references to
• [x] Application passwords always available (even if they are not enabled)

[peterwilsoncc]

I've put this on the 2.0.0 milestone to action early and avoid adding any new code to support older versions of WP or PHP.