tell Scala Steward not to update stuff

as per remarks in comment

.scala-steward.conf

@@ -0,0 +1,10 @@
+updates.ignore = [
+  // As per discussion on https://github.com/scala/scala-collection-compat/pull/579 ,
+  // we agreed not to drop 2.11 support for Scala.js and Scala Native, even though
+  // both of those projects have themselves dropped 2.11 support. We could revisit
+  // the decision if some concrete benefit to upgrading everything comes along, but
+  // for now anyway, we stay frozen at the last versions when 2.11 worked.
+  { groupId = "org.scala-lang", artifactId = "scala-library" },
+  { groupId = "org.scala-lang", artifactId = "scala3-library" },
+  { groupId = "org.scala-native" }
+]