Upgrade test infrastructure to support 'edge' versions of dependencies like MySQL8

@totten wrote:

we currently check releases using a matrix with min and max columns

if we expect civicrm to be compatible already (and/or that updates will be quick and straight-forward), then we can use KISS approach and just bump up max (mysql57 => mysql80)

otoh, if the process is going to take longer, then we should add a new column (min, max, edge)

(note: adding an edge column could also be useful wrt to phasing-in support for php73)

... it would mean that reports of errors for infrastructure that is on the edge of getting support are not treated as blockers, and are identifiable as edge errors

Once an edge technology dependency like MySQL 8 or PHP 7.3 are deemed ready to be declared supported, the max column will change, and the edge column will change to NULL or perhaps the next higher version if people are prepared to start work on that.

This issue is focused on adding edge support to the matrix through a new column, and everything that entails in the build infrastructure and error reporting.

In order to do that, MySQL8 happens to be the first edge version to be incorporated. Issues specific to getting MySQL8.0 testing working on infrastructure will be managed at #1144 (closed). Issues regarding MySQL8 compatibility are being managed via the meta issue #392 (closed).

PHP73 is expected to be the next edge dependency.