Cannot add GitHub Actions Secret with `GITHUB_` prefix

[bfrancom]

Describe the bug

GitHub Actions does not allow any secret or variable to begin with GITHUB_ as those are reserved variable prefixes.

Steps to reproduce the behavior

1. Go to the 'repo->Actions'
2. Click on 'New repository secret' button
3. Try to add secret with GITHUB_ prefix
4. See error

Expected behavior

This may have worked in the past, but is now deprecated and does not work

Screenshots

Additional context

See GitHub docs:
https://docs.github.com/en/actions/learn-github-actions/variables#naming-conventions-for-configuration-variables