[CI] license-header-check doesn't like Markdown files

[lorentey]

The "soundness" check called license-header-check is choking on the file .github/ISSUE_TEMPLATE/BUG_REPORT.md.

▸ Run curl -s https://raw.githubusercontent.com/swiftlang/github-workflows/refs/heads/main/.github/workflows/scripts/check-license-header.sh | bash
** ERROR: Unsupported file extension md for file (exclude or update this script): .github/ISSUE_TEMPLATE/BUG_REPORT.md
bash: line 74: expected_file_header: unbound variable
Error: Process completed with exit code 1.

This check is therefore disabled. Once the issue above is fixed, we can consider enabling it.

[FranzBusch]

You can use a .licenseignore file. We normally ignore all markdown files https://github.com/apple/swift-nio/blob/2a3a333d8b35a0decb8980497a604e51313c4b2a/.licenseignore#L12

So this is not an issue with the soundness workflow but something that you have to do in your repository here.