chg: ci: Migrate Mattermost notifications to Zulip in CI

Since internal communications are now Zulip based, CI jobs now target
Zulip instead of Mattermost. The `MATTERMOST_WEBHOOK_URL` environment
variable is no longer needed, scripts now use `ZULIP_SERVER_URL` and
`ZULIP_API_KEY`.

In order to harmonize Zulip messaging, `message_zulip.py` is used where
curl calls to the webhook were previously used.

Merge branch 'andoni/mattermost-to-zulip-migration' into 'main'

See merge request isc-projects/bind9!12199