ci: Compile with Java 11 and run tests with Java 8

Author: lqiu96

.github/workflows/downstream_protobuf_compatibility_check_nightly.yaml

@@ -47,10 +47,11 @@ jobs:
         uses: actions/checkout@v4
       - uses: actions/setup-java@v4
         with:
-          # TODO(https://github.com/GoogleCloudPlatform/cloud-opensource-java/issues/2395): Java 17+ support for Linkage Checker
           java-version: 8
           distribution: temurin
       - run: echo "JAVA8_HOME=${JAVA_HOME}" >> $GITHUB_ENV
+      # Java Client Libraries are compiled with Java 11 and target Java 8. Java 11 is required because GraalVM
+      # minimum support is for Java 11.
       - uses: actions/setup-java@v4
         with:
           java-version: 11

.kokoro/nightly/downstream-protobuf-binary-compatibility.sh

@@ -64,6 +64,7 @@ function build_program_arguments() {
 
 java -version
 
+# TODO(https://github.com/GoogleCloudPlatform/cloud-opensource-java/issues/2395): Java 17+ support for Linkage Checker
 # cloud-opensource-java contains the Linkage Checker tool
 git clone https://github.com/GoogleCloudPlatform/cloud-opensource-java.git
 pushd cloud-opensource-java