docs: confirm legacy-updater zpg filename convention for pre-5.0 upgrade path

[coretl]

Target page

how-to/upgrade-via-web-admin

The pre-5.0 upgrade path instructs users to download the legacy-updater package
from GitHub Releases, but does not state the exact filename/pattern.

Category: verify-then-write

What a human must do (~1 hour)

1. Check the GitHub Releases page for meta-panda for the legacy zpg/zpkg
   updater packages.
2. Document the exact filename pattern (e.g. panda-upgrade-<version>.zpg)
   and any machine-type suffix.
3. Confirm whether the file extension is .zpg, .zpkg, or something else.
4. Update how-to/upgrade-via-web-admin.md with a concrete filename example.

Post findings as a comment or PR.

Source

docs/how-to/upgrade-via-web-admin.md — the "Pre-5.0 PandAs" subsection,
step 1.