ci: Use Java 8 for Linkage Checker

[lqiu96]

Manual invocation: https://github.com/googleapis/sdk-platform-java/actions/runs/14038997003

[lqiu96]

Two odd class file version issues:

1. Datatstore

Error: Exception in thread "main" java.lang.UnsupportedClassVersionError: com/google/cloud/datastore/emulator/CloudDatastore has been compiled by a more recent version of the Java Runtime (class file version 55.0), this version of the Java Runtime only recognizes class file versions up to 52.0

2. Spanner

Error:  COMPILATION ERROR : 
[INFO] -------------------------------------------------------------
Error:  /home/runner/work/sdk-platform-java/sdk-platform-java/java-spanner/google-cloud-spanner/src/main/java/com/google/cloud/spanner/nativeimage/SpannerFeature.java:[20,38] 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.

[suztomo]

Please confirm using Java 17 in compilation can catch Protobuf 4.30.1's Java 8 incompatibility issue or not.

[lqiu96]

Ah, Downstream tests cannot use Java 8 for compilation: #3717. This is also documented go/cloud-java-native-image#class-file-has-wrong-version-550-should-be-530 (thanks @mpeddada1)

I believe we must compile the client libraries with JDK 11+. Closing this PR. I think we will have to find a different solution to try to compile the client libraries using Protobuf 4.x and JDK 8.

CC: @suztomo

[lqiu96]

This seems a bit odd that the downstream tests failed for all client librarires when using Java 8, but it was able to compile and run unit tests and pass for most handwritten libraries using Java 8 in this PR.

Going to run another invocation to see

[suztomo]

There are failure in the check in this pull request as well as your invocation:

[INFO] Tests run: 3, Failures: 0, Errors: 0, Skipped: 0, Time elapsed: 0.001 s -- in com.google.cloud.datastore.aggregation.CountAggregationTest
[INFO] 
[INFO] Results:
[INFO] 
Error:  Errors: 
Error:    DatastoreTest.afterClass:196 » Connect Connection refused (Connection refused)
Error:    DatastoreTest.setUp:189 » Datastore I/O error

[sonarqubecloud]

Please retry analysis of this Pull-Request directly on SonarQube Cloud

[suztomo]

It passed.

[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