Stop compiling most stuff with Scala 2 (#1113)

Author: alexarchambault

.scalafmt.conf

@@ -12,7 +12,7 @@ newlines.beforeMultiline = keep
 newlines.afterCurlyLambdaParams = keep
 newlines.alwaysBeforeElseAfterCurlyIf = true
 
-runner.dialect = scala213source3
+runner.dialect = scala3
 
 rewrite.rules = [
   RedundantBraces
@@ -31,26 +31,6 @@ rewrite.sortModifiers.order = [
   "implicit", "sealed", "abstract", "lazy"
 ]
 
-fileOverride {
-  "glob:**/scala-3-stable/**" {
-    runner.dialect = scala3
-  }
-  "glob:**/scala-3-unstable/**" {
-    runner.dialect = scala3
-  }
-  "glob:**/sclicheck/**" {
-    runner.dialect = scala3
-  }
-  "glob:**/gifs/**" {
-    runner.dialect = scala3
-  }
-  "glob:**/scala-3.1/**" {
-    runner.dialect = scala3
-  }
-  "glob:**/negative-tests/**" {
-    runner.dialect = scala3
-  }
-}
 project.excludeFilters = [
   ".bloop"
   ".metals"

README.md

@@ -30,7 +30,7 @@ The Scala CLI sources ship with Mill launchers, so that Mill itself doesn't need
 #### Run unit tests
 
 ```bash
-./mill 'build-module[_].test'
+./mill 'build-module.test'
 ```
 
 #### Run integration tests with the JVM launcher