Minimum PHP version bump

[ernilambar]

Current: 7.2

Sparked from #789

I am working on updating the readme parser to use its latest version. That class is coming from WP.org and its minimum PHP support is 7.4. So we are now stuck the older version of the readme parser in our plugin.

I created ticket in meta about the issue but it did not get traction to make parser class to make PHP 7.2 compatible - https://meta.trac.wordpress.org/ticket/7749

So, may be we could bump our min PHP version instead of using composer patch for the parser.

I have not seen the discussion about minimum PHP version support in PCP particularly. What philosophy we are following for the minimum PHP version support?