Dropping legacy version support

We have a long legacy tail stretching back to PHP 5.6 right now.
This is preventing us from using many new features, as well as necessitating a CI build which requires a dated version of OpenSSL (currently not working).

At the minimum I think we should drop PHP 5.6, and I would be in favour of dropping 7.x as well. These versions are long since obsolete.

@bukka thoughts?