[8.19] Platform-independent paths in TestBuildInfoPluginFuncTest (#128303)

[prdoyle]

Backports the following commits to 8.19:

• Platform-independent paths in TestBuildInfoPluginFuncTest (Platform-independent paths in TestBuildInfoPluginFuncTest #128303)

[prdoyle]

This is failing because the code is not compatible with Java 17.

Caused by: org.gradle.api.internal.tasks.compile.CompilationFailedException: Compilation failed; see the compiler output below.
--
  | /dev/shm/bk/bk-agent-prod-gcp-1748008317112679671/elastic/elasticsearch-pull-request/build-tools/src/main/java/org/elasticsearch/gradle/plugin/GenerateTestBuildInfoTask.java:222: error: cannot find symbol
  | versions = versions.reversed();
  | ^
  | symbol:   method reversed()
  | location: variable versions of type List<Integer>
  | /dev/shm/bk/bk-agent-prod-gcp-1748008317112679671/elastic/elasticsearch-pull-request/build-tools/src/main/java/org/elasticsearch/gradle/plugin/GenerateTestBuildInfoTask.java:321: error: cannot find symbol
  | File find = files.removeFirst();
  | ^
  | symbol:   method removeFirst()
  | location: variable files of type List<File>
  | 2 errors

This PR eliminates the call to List.removeFirst, so that will help a bit.