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

Author: lqiu96

.github/workflows/downstream_protobuf_compatibility_check_nightly.yaml

@@ -50,6 +50,12 @@ jobs:
           # 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}" >> $GITHUB_ENV
+      - uses: actions/setup-java@v4
+        with:
+          java-version: 11
+          distribution: temurin
+      - run: echo "JAVA11_HOME=${JAVA_HOME}" >> $GITHUB_ENV
       - name: Print Protobuf-Java testing version
         run: echo "Testing with Protobuf-Java v${{ matrix.protobuf-version }}"
       - name: Perform downstream source compatibility testing

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

@@ -62,6 +62,8 @@ function build_program_arguments() {
   done
 }
 
+java -version
+
 # cloud-opensource-java contains the Linkage Checker tool
 git clone https://github.com/GoogleCloudPlatform/cloud-opensource-java.git
 pushd cloud-opensource-java

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

@@ -20,6 +20,14 @@ source "${scriptDir}/common.sh"
 
 validate_protobuf_compatibility_script_inputs
 
+
+java -version
+# Compile the repos with Java 11
+mvn clean compile -T 1C
+
+export JAVA_HOME="${JAVA8_HOME}"
+java -version
+
 # REPOS_UNDER_TEST Env Var accepts a comma separated list of googleapis repos to test. For Github CI,
 # this will be a single repo as Github will build a matrix of repos with each repo being tested in parallel.
 # For local invocation, you can pass a list of repos to test multiple repos together.
@@ -32,7 +40,7 @@ for repo in ${REPOS_UNDER_TEST//,/ }; do # Split on comma
   # Run unit tests to help check for any behavior differences (dependant on coverage)
   if [ "${repo}" == "google-cloud-java" ]; then
     # The `-am` command also builds anything these libraries depend on (i.e. proto-* and grpc-* sub modules)
-    mvn clean test -B -V -ntp \
+    mvn test -B -V -ntp \
       -Dclirr.skip \
       -Denforcer.skip \
       -Dmaven.javadoc.skip \
@@ -41,7 +49,7 @@ for repo in ${REPOS_UNDER_TEST//,/ }; do # Split on comma
       -pl "${google_cloud_java_handwritten_maven_args}" -am \
       -T 1C
   else
-    mvn clean test -B -V -ntp \
+    mvn test -B -V -ntp \
       -Dclirr.skip \
       -Denforcer.skip \
       -Dmaven.javadoc.skip \