Deprecate the fallback DNS pool

When: after Tails 6.0 is out, so that every Tails 5.x that predates !983 (merged) can still fallback to the DNS pool on failure, when downloading an automatic upgrade to a newer 5.y.

• Updates needed include:
  • Mirrors design docs: wiki/src/contribute/design/
  • Mirror contributors' doc: wiki/src/contribute/how/mirror.mdwn
  • Sysadmin services doc: wiki/src/contribute/working_together/roles/sysadmins/services.mdwn
  • Automated tests (features/): stop using the fallback DNS pool (migrate to another hostname that has multiple A records)
• Remove the mirrors.git repo and the corresponding hook & SSH access
  • Note: the mirror-pool repo is still necessary.
• Add redirection on our web server, for backwards compatibility: http://dl.amnesia.boum.org → https://download.tails.net
• Point the dl.amnesia.boum.org DNS record to our web server
• Notify admins of mirrors.edge.kernel.org that they can ditch their Tails mirror.