Simplify Scala workflow

[Jentsch]

Before this change the local development used
a different version (2.12.18) of scala than the CI.

[Atry]

This project ignores local.sbt so that you can specify the scalaVersion in it for local development, allowing for a diiferent version from CI. It was on purpose to not specify it in build.sbt.

sbt-example/.gitignore

Line 2 in 0dcb036

local.sbt

[Jentsch]

Sorry, I'd saw that it was intentional. But it still works, accidentally I would argue, because sbt loads build files in alphabetical order to this day. If you create a local.sbt with scalaVersion := "2.12.18" it overrides the configuration in the build.sbt.

$ sbt show scalaVersion
[info] loading settings for project sbt-example from build.sbt,local.sbt,secret.sbt ...
[info] set current project to sbt-example (in build file:/workspaces/sbt-example/)
[info] 2.12.18

That mechnis isn't well documentent and quite implicit, so I would be happy to drop the last commit. What do you think?