Add PHP 8 support

[weierophinney]

This patch builds on #7; I have resubmitted it because a bug in the CI matrix job and how it performs checkouts leads to a branch collision.

• php 8 support. also drop support for php <7.3
• phpunit 9 support and some undefined properties
• phpunit 9 prep changes and some undefined properties
• constrain laminas/laminas-coding-standard to v1 as before
• update tests with deprecated methods or lack of assertions
• remove obsolete CI configuration

Closes #7 Fixes #6

[weierophinney]

Closing; tests ran on #7 finally.