Update securedrop-keyring package for TemplateVMs

[conorsch]

Towards freedomofpress/securedrop#5923. In order to rotate the SecureDrop Release Signing Key on the workstation, we must bump the public keyring in the securedrop-keyring package, used in the Debian-based VMs. The package config is here: https://github.com/freedomofpress/securedrop-debian-packaging/tree/963db4100e0dfb5d5ccc019e8b70afe4441f6cab/securedrop-keyring

Note that there's a separate securedrop-keyring package for the servers, changes for which are visible in freedomofpress/securedrop#5930. The same changes should be replicated in the workstation package: version bump, and import new pubkey into the existing keyring, so two pubkeys are present.

[eloquence]

(Transferred into this repo since the package config lives here.)

[eloquence]

The package was updated via #250; keeping issue open until release (which can of course also be split into a follow-up issue).

[conorsch]

As mentioned in freedomofpress/qubes-template-securedrop-workstation#20 (comment), the 0.1.5 keyring packaged has been released to prod, so closing.