dep: update mill to a stable version in community build

This updates mill to the latest stable release 0.10.5.

Author: ckipp01

community-build/src/scala/dotty/communitybuild/projects.scala

@@ -99,7 +99,7 @@ final case class MillCommunityProject(
     // uncomment once mill is released
     // if ignoreDocs then null else s"$baseCommand.docJar"
   override val runCommandsArgs = List("-i", "-D", s"dottyVersion=$compilerVersion")
-  override val environment = Map("MILL_VERSION" -> "0.9.6-16-a5da34")
+  override val environment = Map("MILL_VERSION" -> "0.10.5")
 
 final case class SbtCommunityProject(
     project: String,