Deprecate the Tails Upgrader for Tails versions 5.13 and older

Since redirection from tails.boum.org to tails.net was implemented (#17985 (closed)) we maintain exceptions without which some components would be broken in older images that use code that doesn't support redirects:

Path	Impacted component	Tails version
/mirrors.json	Tails Upgrader	≤ 5.8
/upgrade/*/upgrades.yml	Tails Upgrader	≤ 5.13
/tails-*.key	Tails Upgrader	≤ 5.13
/security/index.*.atom	tails-security-check	≤ 5.13

Note 1: Redirecting the above would also break the "You should upgrade" notification, in addition to upgrades themselves. When this happens, we point users to file:///usr/share/doc/tails/website/doc/upgrade/error/check.en.html, which we can't improve retroactively either.

Note 2: The page shown on upgrade check failures has instructions for upgrades for 4.15-1.18 with a curl command that used to use the old domain to fetch the CA certificate. Despite that, I checked and that page was not included in any of the affected versions, so I think adding an exception for that redirection would have no practical effect.

Potential plan

• Schedule a date and announce the future deprecation of the Tails Upgrader in Tails versions 5.13 and older.
• Wait long enough that we're comfortable breaking the Upgrader for old releases. (1 year?)
• Remove redirection exceptions.
• Announce the implementation of the deprecation.