Use the latest 2.1.x for now, not 2.2.x (#6373)

Author: BoD

scripts/bump-kotlin-nightlies.main.kts

@@ -8,7 +8,7 @@ val BRANCH_NAME = "kotlin-nightlies"
 
 fun bumpVersions() {
   val kotlinVersion =
-    getLatestVersion("https://maven.pkg.jetbrains.space/kotlin/p/kotlin/dev/org/jetbrains/kotlin/kotlin-stdlib/maven-metadata.xml")
+    getLatestVersion("https://maven.pkg.jetbrains.space/kotlin/p/kotlin/dev/org/jetbrains/kotlin/kotlin-stdlib/maven-metadata.xml", prefix = "2.1")
   val kspVersion =
     getLatestVersion("https://oss.sonatype.org/content/repositories/snapshots/com/google/devtools/ksp/com.google.devtools.ksp.gradle.plugin/maven-metadata.xml")
   File("gradle/libraries.toml").let { file ->