activity-report-2024: Mention that it’s the 7th report.

* drafts/activity-report-2024.md <intro>: Add “seventh”.

drafts/activity-report-2024.md

@@ -29,7 +29,7 @@ the years, we have reached out to many organizations and people who
 share our goal of improving upon the status quo when it comes to
 software deployment.
 
-This report highlights key achievements of Guix-HPC between [our
+This report—our seventh report!—highlights key achievements of Guix-HPC between [our
 previous
 report](https://hpc.guix.info/blog/2024/02/guix-hpc-activity-report-2023/)
 a year ago and today, February 2025.  This year was marked by exciting