jobs/template/debian: Update cassandra.asc

The current one is only one key.

This is to fix the current debian build failure of the storage module [1]

That inlines [2] as our cassandra.asc file used within the build.

• [1] https://jenkins.softwareheritage.org/view/Debian%20packages/job/debian/job/packages/job/DSTO/job/gbp-buildpackage/162/console

• [2] https://downloads.apache.org/cassandra/KEYS

Test Plan

tox

---

Migrated from D3110 (view on Phabricator)