RC: update instructions for new atlas_git_merge.py

Update instructions for the new release branch merge script (atlas/athena!78834 (merged)).

cc @atlas-release-coordinators

added 3 commits

• c584f533...9512772e - 2 commits from branch main
• dd4a993a - RC: update instructions for new atlas_git_merge.py

Compare with previous version

mentioned in commit 0396fd21

merged