Give some love to the project

- Remove sbt 1.0.0 support
- Update Scala version
- Update sbt and sbt plugins
- Remove deprecated sbt syntax usages
- Update Scalafix and improve its configuration
- Install Scala Steward to help us maintain this project
- Fix Scalafix issues in code

build.sbt

@@ -43,7 +43,6 @@ lazy val plugin = project
   .settings(
     moduleName := "sbt-native-image",
     sbtPlugin := true,
-    pluginCrossBuild / sbtVersion := "1.0.0",
     crossScalaVersions := List(scala212),
     buildInfoPackage := "sbtnativeimage",
     buildInfoKeys := Seq[BuildInfoKey](version),