Fixes link URL for JUnit XML format

What does this MR do and why?

The link for the JUnit XML format in the IBM documentation for z/OS is broken as it points to an older version (14.1.x) that is no longer published.

This MR updates the link URL to the latest version (16.0) of the IBM documentation in the following topics:

• https://docs.gitlab.com/ee/ci/testing/unit_test_reports.html#how-it-works
• https://docs.gitlab.com/ee/ci/yaml/artifacts_reports.html#artifactsreportsjunit

Relates to issue: #299086 (comment 1747445412)