Downgrade Siouan plugin

Author: jonolehagemo

gradle/libs.versions.toml

@@ -15,7 +15,7 @@ shadow-version = "8.3.8"
 gradle-git-properties-version = "2.5.4"
 devtools-ksp-version = "2.3.3"
 micronaut-application-version = "4.6.1"
-siouan-frontend-jdk21-version = "10.0.0"
+siouan-frontend-jdk21-version = "9.1.0"
 # to be removed
 assertj-core-version = "3.27.6"