actually use JDK21 for JDK21 CI

https://docs.scala-lang.org/overviews/jdk-compatibility/overview.html
- ignore scripts with hardcoded sbt versions < 1.9.0
- don't load artifacts built with old scala versions

.github/workflows/ci.yml

@@ -37,10 +37,10 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v4
-      - uses: olafurpg/setup-scala@v14
+      - uses: coursier/setup-action@v1
         with:
-          jvm: temurin:17
-      - run: sbt "test; scripted sbt-*/* skip-windows/*"
+          jvm: temurin:21
+      - run: sbt "test; scripted sbt-scalafix/* skip-windows/*"
   windows:
     name: Windows tests
     runs-on: windows-latest