generate_milestone_reports.py: Add M124

Bug: None
Change-Id: Ic7dc42d231e1955fac3a480793a0c96642ed5ae8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5409299


Commit-Queue: Sam Maier <smaier@chromium.org>
Auto-Submit: Andrew Grieve <agrieve@chromium.org>
Reviewed-by: Sam Maier <smaier@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1281138}