Change GitHub Actions version to JDK 8 (CaffeineMC#276)

More and more users are using GitHub actions, and running into CaffeineMC#16. This is probably the most efficient way to prevent complaints before they happen.

.github/workflows/gradle.yml

@@ -8,10 +8,10 @@ jobs:
 
     steps:
     - uses: actions/checkout@v2
-    - name: Set up JDK 14
+    - name: Set up JDK 8
       uses: actions/setup-java@v1
       with:
-        java-version: 14
+        java-version: 8
     - name: Grant execute permission for gradlew
       run: chmod +x gradlew
     - name: Build with Gradle
@@ -20,4 +20,4 @@ jobs:
       uses: actions/upload-artifact@v1
       with:
         name: build-artifacts
-        path: build/libs
+        path: build/libs