* saltstack-formulas/prometheus-formula#67

Justification given here:

* saltstack-formulas/prometheus-formula#60 (comment)

* https://gitlab.com/saltstack-formulas/infrastructure/salt-image-builder/-/merge_requests/114

* https://gitlab.com/saltstack-formulas/infrastructure/salt-image-builder/-/merge_requests/115

Introducing this so that Debian 11 can be introduced now throughout all
formulas.  For the most part, these instances are only failing because
the upstream repo hasn't been made available just yet; rather than
disabling Debian 11 instances, let them run and then use them as
standard instances once they start passing.

* Debian 11, awaiting upstream repo:
  - `nginx-formula` (`passenger` suite)
  - `node-formula`
  - `packages-formula` (which fails due to the dependency on the `node-formula`)
  - `powerdns-formula`
  - `rspamd-formula`
  - `salt-formula`
  - `telegraf-formula`
  - `varnish-formula`
  - `zabbix-formula`

* Debian 11, service enabling issue:
  - `stunnel-formula`