Disable Autocrypt even for existing persistent Thunderbird profiles
See #16223 (comment 15143): people who upgraded through 3.10 will still have Autocrypt enabled and might send unencrypted emails unwillingly.
We could have a migration script that automatically applies the default configuration from Autocrypt 3.11 (Autocrypt disabled + acPreferEncrypt) when we detect the configuration we had in 3.10 (Autocrypt enabled + no acPreferEncrypt).
We could keep this migration script around during some release cycles and document that if people want to enable Autocrypt they have to enable acPreferEncrypt to avoid triggering the migration script.
Feature Branch: hefee/bugfix/16186-disable-autocrypt+force-all-tests
Parent Task: #15923 (closed)