Documentation updates for SecureDrop 2.8.0

[eloquence]

This is a tracking ticket to document changes pertaining to SecureDrop 2.8.0.

• [x] Tails 6 updates: https://github.com/freedomofpress/securedrop-docs/pull/538
• [x] Release boilerplate with reminder to migrate to Tails 6
• [x] Tails 6 migration instructions

[zenmonkeykstop]

This was completed