Bump minimum PHP version to 7.2

[johnbillion]

Query Monitor 4.0 will bump the minimum required version of PHP to 7.2. This facilitates that change, removes various back-compat code, and modernises some functionality.

Entry points will remain compatible with PHP 5.3 to avoid causing a fatal error with newer syntax. A PHP upgrade message will be shown in the admin area (it's already there for PHP < 5.3).