Delete outdated sub-ecosystem data inosv-vulnerabilities bucket

[hogo6002]

The osv-vulnerabilities bucket currently stores all vulnerabilities exported by the exporter, including historical entries (no auto deletion). We recently modified the exporter to only export vulnerabilities for the main ecosystem (e.g. Debian, Ubuntu) instead of individual sub-ecosystems (e.g. Debian:11, Debian:12). Because of this change, a large number of legacy sub-ecosystem directories remain there.

To maintain a cleaner bucket, we should remove these outdated directories and files.

If we also want to remove nonexistent vulnerability records from each ecosystem, this could resolve issues like #2902

[andrewpollock]

I think we can break this into two pieces of work:

1. a one-time manual cleanup of all of the directories with colons in the name
2. engineering to the exporter for individual record cleaning, as described in exporter: individual records no longer in existence should be removed from the GCS export #2902