Github actions - compile with Java 8, generate JavaDoc with Java 21.

.github/workflows/main.yml

@@ -33,11 +33,18 @@ jobs:
     steps:
     - name: Checkout
       uses: actions/checkout@v3
-    - name: Set up JDK 21
-      uses: actions/setup-java@v3
+    - name: Set up JDK 8
+      uses: AdoptOpenJDK/install-jdk@v1
       with:
-        java-version: 21
-        distribution: temurin
+        version: '8'
+        architecture: x64
+        targets: 'JDK_8;JAVA_HOME'
+    - name: Set up JDK 21
+      uses: AdoptOpenJDK/install-jdk@v1
+      with:
+        version: '21'
+        architecture: x64
+        targets: 'JDK_21'
     - name: Set up PHP
       run: |
         apt update -y -qq