Increase minimum PHP requirement to 7.4?

[remcotolsma]

As far as I'm concerned we require PHP 7.4, this is already the recommendation from WordPress: https://wordpress.org/about/requirements/. PHP 7.3 only has security support until 6 dec 2021: https://www.php.net/supported-versions.php.

Originally posted by @remcotolsma in #271 (comment)

[rvdsteege]

A roughly estimated 70% of Pronamic Pay users seem to use PHP version >= 7.4, based on support (YTD). The current percentage is probably a little higher.

As active support for PHP 7.4 ends in 3 weeks and WordPress also recommends PHP version 7.4, I think it's fine to bump the minimum PHP version requirement to 7.4.

[remcotolsma]

Increased it in 076cded. As soon as we work on extensions and gateways, we can also implement the increase there. No need to apply it everywhere at once.