Bump minimum support versions to WordPress 5.6 and PHP 7.2

[GeekPress]

Describe the solution you'd like We would like to bump the minimum required WordPress version to 5.6 and PHP version to 7.2.

We have today 98.28% of our users using WordPress 5.6 or + and 99.33% of our users using PHP 7.2 or +.

What do we need to change? In wp-rocket.php, update WP_ROCKET_WP_VERSION to 5.6 and WP_ROCKET_PHP_VERSION to 7.2

We also need to update test matrix in GitHub workflows.