Correct the archive count claim in the offline malware advisory docs

What does this MR do and why?

The offline malware advisory procedure said the archive count is "read from checkpoint.json, up to a maximum of 64 named 00 through 3f". Nothing reads a count from that file. The script writes only {until: $until}, OfflineV3#full_dataset_until reads only parsed['until'], and the shard list comes from globbing *.tar.zst. The 64 ceiling is a service-side choice rather than a documented guarantee, and the licenses dataset already publishes 128 from the same exporter, so the doc now describes the naming and leaves the number to the service.

Also drops "Running it on a schedule therefore costs one small request per registry", which restates the sentence in front of it.

I checked the rest of the section against master while I was in here. The jitter paragraph, the 22 minute lease and its Cannot obtain an exclusive lease. message at ERROR, the files_ingested and storage_type log fields, the Malware advisory upsert skipped: ingest_malware_advisories disabled string, the model and constant names in the Rails console commands, and the registry-identifier renames are all accurate and unchanged.

Relates to

• #611230