Upgrade community_build_a to JDK 16

Fixes #16423

Author: nicolasstucki

.github/workflows/ci.yaml

@@ -264,8 +264,8 @@ jobs:
          )"
 
     steps:
-      - name: Set JDK 15 as default
-        run: echo "/usr/lib/jvm/java-15-openjdk-amd64/bin" >> $GITHUB_PATH
+      - name: Set JDK 16 as default
+        run: echo "/usr/lib/jvm/java-16-openjdk-amd64/bin" >> $GITHUB_PATH
 
       - name: Reset existing repo
         run: git -c "http.https://github.com/.extraheader=" fetch --recurse-submodules=no "https://github.com/lampepfl/dotty" && git reset --hard FETCH_HEAD || true