sbt-scalafix: drop support for sbt 0.13

[bjaglin]

It is probably a good time to drop support for sbt 0.13 - sbt 1.0 was released 3,5 years ago.

Benefits/TODO:

• use scala 2.12 features
• get rid of the 1.0/0.13 compatibility layer
• use slash notation (in code and in docs)
• restructure scripted to run most (all?) of them against different sbt 1.x versions, like test sbt compatibility via GH matrix rather than dedicated scripted thesamet/sbt-protoc#196

[bjaglin]

Considering that it most likely won't be particularly harder to ship #1146 with sbt 0.13 support, I think it would be nice to hold this just a little longer, so that clients stuck to sbt 0.13 can still benefit from future versions of scalafix itself. Once that's done, then I think it's really safe to drop support and leverage newer sbt features.

[mlachkar]

We can maybe close this ticket now ?

[bjaglin]

Why? The work hasn't started.

[mlachkar]

ok. I thought we dropped support of sbt 0.13 in sbt-scalafix, but I am wrong. scalacenter/sbt-scalafix#219
We dropped support of 0.13 for testkit only.