Stop supporting Scala 2.12

[geirolz]

Opening this ticket to talk about scala 2.12 support. I wonder when and if we want to stop supporting it - this creates some limitations due the compatibility and more maintenance work

Should we consider to stop supporting this version ?

[matejcerny]

From the current list of adopters: @orium @ybasket @sentenza

[ybasket]

Thank you for the ping! The adopters list is outdated (will send a PR): Free2Move no longer exists and hence doesn't use fs2-rabbit. So go ahead dropping 2.12 👍

[geirolz]

Planned for version 5.4.0

[matejcerny]

Planned for version 5.4.0

Please add some exclamation marks in the release description 😄