Add sbt caching to GitHub Actions

Author: sim642

.github/workflows/sbt.yml

@@ -12,10 +12,11 @@ jobs:
     steps:
     - uses: actions/checkout@v4
     - name: Set up JDK 21
-      uses: actions/setup-java@v2
+      uses: actions/setup-java@v4
       with:
         distribution: 'zulu'
         java-version: '21'
+        cache: 'sbt'
     - uses: sbt/setup-sbt@v1
     - name: Test with SBT
       run: sbt test