configure scala-steward message and pin to scala3 LTS

Author: rayrobdod

.github/scala-steward.conf

@@ -0,0 +1,8 @@
+commits.message = "[build] ${default}"
+
+pullRequests.customLabels = [ "library-update" ]
+
+updates.pin = [
+  { groupId = "org.scala-lang", artifactId="scala3-library", version = "3.3." },
+  { groupId = "org.scala-lang", artifactId="scala3-library_sjs1", version = "3.3." },
+]