Add MySQL 8, Ubuntu 20.04 and EL8 support

[EMH333]

Description

• Adds support for MySQL 8
• Adds support for Ubuntu 20.04
• Adds support for CentOS 8

Also removes EL6 and Ubuntu 14.04 both of which are EOL/close to EOL

Issues Resolved

Related to #611

This also is a prerequisite for #522 (for which I will have another PR very soon)

Supersedes #584 and #626

Check List

• [x] All tests pass. See https://github.com/sous-chefs/mysql/blob/master/TESTING.md
• [x] New functionality includes testing.
• [x] New functionality has been documented in the README if applicable

[xorima]

Hey @EMH333

This is awesome!

Can you get something added to the changelog as well please though

Thanks!

[EMH333]

Done!

[xorima]

Can we also have documented the versions of mysql that get removed by this pr in the changelog, just so no one is caught by surprise :)

[kitchen-porter]

Released as: 9.0.0