Proposal: drop support for PHP 5.6 and move to at least 7.2

Since PHP support for 5.6 has stopped long ago and even version 7.1 only has security support (until December 1st), I would like to propose to set the required PHP version to >=7.2. This would, allow for some clean up of code, enable more type hinting, etc.

Please let me know if other people would like move forward too or if there are any reasons why not to do this.