test: add unit test for building with java 11 and testing against java 8

.github/workflows/ci.yaml

@@ -25,7 +25,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        java: [8, 11, 17, 21, 25]
+        java: [11, 17, 21, 25]
     steps:
     - name: Get current week within the year
       id: date
@@ -45,6 +45,39 @@ jobs:
       env:
         JOB_TYPE: test
         JOB_NAME: units-${{matrix.java}}
+  units-8-runtime:
+    runs-on: ubuntu-latest
+    name: "units (8)"
+    steps:
+    - name: Get current week within the year
+      id: date
+      run: echo "::set-output name=week_of_year::$(date +'%W' --utc)"
+    - uses: actions/checkout@v4
+    - uses: actions/setup-java@v4
+      with:
+        java-version: 11
+        distribution: temurin
+        cache: maven
+    - run: java -version
+    - uses: actions/cache@v4
+      id: mvn-cache
+      with:
+        path: ~/.m2/repository
+        key: ${{ runner.os }}-maven-unified-${{ steps.date.outputs.week_of_year }}
+    - name: Install all modules using Java 11
+      shell: bash
+      run: |
+        mvn -V -B -ntp clean install -DskipTests -Dfmt.skip -Dclirr.skip -Denforcer.skip -Dcheckstyle.skip -Dflatten.skip -Danimal-sniffer.skip -Djacoco.skip
+    - uses: actions/setup-java@v4
+      with:
+        java-version: 8
+        distribution: temurin
+    - run: java -version
+    - name: Run tests in Java 8 with the source compiled in Java 11
+      run: .kokoro/build.sh
+      env:
+        JOB_TYPE: test
+        JOB_NAME: units-11-runtime-8
   windows:
     runs-on: windows-latest
     steps: