Update actions/cache action to v6

Author: renovate[bot]

.github/workflows/ci.yml

@@ -39,7 +39,7 @@ jobs:
           java-version: 11
 
       - name: Cache sbt
-        uses: actions/cache@v2
+        uses: actions/cache@v6
         with:
           path: |
             ~/.sbt
@@ -85,7 +85,7 @@ jobs:
           java-version: 11
 
       - name: Cache sbt
-        uses: actions/cache@v2
+        uses: actions/cache@v6
         with:
           path: |
             ~/.sbt