Improve prepare_release_notes.py

Improve prepare_release_notes.py:

• ignore "commented" lines as I got some merge conflicts strings in the log which broke the parsing
• pre-load labels as it seemed to help

To reproduce:

./Build/AtlasBuildScripts/prepare_release_notes.py release/25.0.10 nightly/main/2024-07-02T2101 -t <token> -v

/cc @atlas-release-coordinators