Dropped support EL7, Ubuntu 18.04, Debian 9 and Ruby 2.7

[FooBarWidget]

We've dropped support for a bunch of old distributions as well as Ruby 2.7.

• Enterprise Linux 7's Maintenance Support phase 1 has ended. Besides, newer Ruby versions don't compile on EL7 so it's no longer worth the hassle to support.
• Ubuntu 18.04's standard support has ended.
• Debian 9 has reached End Of Life.
• Ruby 2.7 has reached End Of Life and also doesn't compile on newer distributions.