scala-steward: move Scala 3 pin from 3.3 LTS to 3.9 LTS

Author: neko-kai

.scala-steward.conf

@@ -1,8 +1,8 @@
 updates.pin  = [
-  # use only Scala3 3.3 LTS
-  { groupId = "org.scala-lang", artifactId="scala3-compiler", version = "3.3." },
-  { groupId = "org.scala-lang", artifactId="scala3-library", version = "3.3." },
-  { groupId = "org.scala-lang", artifactId="scala3-library_sjs1", version = "3.3." }
+  # use only Scala3 3.9 LTS (planned Q2 2026, requires JDK 17+)
+  { groupId = "org.scala-lang", artifactId="scala3-compiler", version = "3.9." },
+  { groupId = "org.scala-lang", artifactId="scala3-library", version = "3.9." },
+  { groupId = "org.scala-lang", artifactId="scala3-library_sjs1", version = "3.9." }
 ]
 
 pullRequests.frequency = "@monthly"