Use Scala major versions rather than patch for generated actions

[mdedetrich]

When generating the various github action workflows sbt-github-actions uses the full Scala version (i.e. https://github.com/sbt/sbt-github-actions/blob/main/.github/workflows/ci.yml#L26). sbt now supports running a build against a specific Scala major version instead of the full patch version with the .x syntax, i.e. sbt ++2.12.x will load the sbt build with the the Scala 2.12 version defined by scalaVersion sbt setting.

Ultimately this solution would make the generated github action workflows less brittle when Scala patch versions are updated (i.e. no change is necessary). In turn this also makes it easier to add github status checks for workflows that Involve a Scala version since its only sensitive to epic/major instead of the whole version.