Consider EOL Warning in Updater GUI

[eaon]

Users still using Qubes OS R4.0 may be unaware that their machine is not supported anymore. Since we enforce running the updater anyway, I think that would be the most obvious place to inform users of this fact, prompting them to get in touch with their admins to upgrade ASAP.

Such a screen should probably distinguish between EOL date is coming up and EOL date has passed

[eloquence]

@eaon and I chatted a bit about this today - quick recap:

• We agreed that this is not something that we need to push out urgently, as we have a pretty high touch relationship with the current beta users and can ensure that everyone's migrated to 4.1. So this is more a long term consideration.
• This could be a screen that is displayed prior to the introductory screen (https://github.com/freedomofpress/securedrop-workstation/blob/main/launcher/sdw_updater_gui/strings.py#L2) which is informational, i.e. the user would still be able to continue.
• However, the screen ought to be displayed even if the updater would not otherwise run because the updater has already run within the 8 hour default interval.
• This would only be relevant if Qubes does not create an upstream equivalent, so worth checking in with them on their ongoing work to improve the update experience before implementing something on our end.

[zenmonkeykstop]

Still valid, especially outside of a pilot phase where we may have less/no communication with users.