Change millBinPlatform to 0.13 (#4607)

This will unblock lib maintainers to start supporting M versions

build.mill

@@ -311,13 +311,12 @@ def millDownloadUrl = Task {
 }
 
 def millBinPlatform: T[String] = Task {
-  // val tag = millLastTag()
-  // if (tag.contains("-M")) tag
-  // else {
-  //  val pos = if (tag.startsWith("0.")) 2 else 1
-  //  tag.split("[.]", pos + 1).take(pos).mkString(".")
-  // }
-  "0.11"
+  val tag = millLastTag()
+  if (tag.contains("-M")) tag
+  else {
+    val pos = if (tag.startsWith("0.")) 2 else 1
+    tag.split("[.]", pos + 1).take(pos).mkString(".")
+  }
 }
 
 def baseDir = build.millSourcePath

example/extending/plugins/7-writing-mill-plugins/build.mill

@@ -194,7 +194,7 @@ compiling 1 Scala source...
 > sed -i.bak 's/0.0.1/0.0.2/g' build.mill
 
 > ./mill myplugin.publishLocal
-Publishing Artifact(com.lihaoyi,myplugin_mill0.11_3,0.0.2) to ivy repo...
+Publishing Artifact(com.lihaoyi,myplugin_millSNAPSHOT_3,0.0.2) to ivy repo...
 
 */
 // Mill plugins are JVM libraries like any other library written in Java or Scala. Thus they