Fix wrong names in "merge release to main" automation

[cprecioso]

Fixes some names:

• Envs for CLI GITHUB_REPOSITORY -> GH_REPO (same with GH_TOKEN), those are the correct ones for the gh cli
• File was named .yml instead of .yaml for consistency
• Renamed the GITHUB_ACTOR -> USERNAME env so it doesn't get confused with the gh CLI environment variables

[FranjoMindek]

We also changed more e.g. GITHUB_ACTOR to USERNAME, and same for token?
All is intended?

[cprecioso]

You're right, I wrote the description a little too fast. All of the changes are intended, updated the description.