break(github): Change github_failed_only output suffix to .md

[turingbeing]

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

Description

Please include a summary of the change and which issue is fixed. Please also include relevant motivation and context. List any dependencies that are required for this change.

Updates Markdown file extension from txt --> md

Fixes #3558

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my feature, policy, or fix is effective and works
• New and existing tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules

[gruebel]

thanks 🙂