ci: Use Java 8 for downstream tests

[lqiu96]

Testing based on #3716 (comment)

[sonarqubecloud]

Quality Gate passed for 'gapic-generator-java-root'

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[sonarqubecloud]

Quality Gate passed for 'java_showcase_integration_tests'

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[lqiu96]

Error:

Error:  COMPILATION ERROR : 
Error:  /home/runner/work/sdk-platform-java/sdk-platform-java/gax-java/gax/src/main/java/com/google/api/gax/nativeimage/NativeImageUtils.java:[38,38] cannot access org.graalvm.nativeimage.hosted.Feature
  bad class file: /home/runner/.m2/repository/org/graalvm/sdk/nativeimage/24.1.2/nativeimage-24.1.2.jar(org/graalvm/nativeimage/hosted/Feature.class)
    class file has wrong version 55.0, should be 52.0
    Please remove or make sure it appears in the correct subdirectory of the classpath.
Error:  Failed to execute goal org.apache.maven.plugins:maven-compiler-plugin:3.13.0:compile (default-compile) on project gax: Compilation failure
Error:  /home/runner/work/sdk-platform-java/sdk-platform-java/gax-java/gax/src/main/java/com/google/api/gax/nativeimage/NativeImageUtils.java:[38,38] cannot access org.graalvm.nativeimage.hosted.Feature
Error:    bad class file: /home/runner/.m2/repository/org/graalvm/sdk/nativeimage/24.1.2/nativeimage-24.1.2.jar(org/graalvm/nativeimage/hosted/Feature.class)
Error:      class file has wrong version 55.0, should be 52.0
Error:      Please remove or make sure it appears in the correct subdirectory of the classpath.
Error:  -> [Help 1]