Bump WordPress and PHP minimum

[peterwilsoncc]

Description of the Change

Bump WordPress minimum to 5.7 Bump PHP minimum to 7.4

Closes #905, #925

How to test the Change

Changelog Entry

Changed - Now requires PHP 7.4 or later and WordPress 5.7 or later.

Credits

Props @peterwilsoncc, @vikrampm1.

Checklist:

• [x] I agree to follow this project's Code of Conduct.
• [x] I have updated the documentation accordingly.
• [ ] I have added tests to cover my change.
• [ ] All new and existing tests pass.