Migration from Mailman 2 to Mailman 3

[Firefishy]

Mailman 2 is no longer supported on current versions of Ubuntu and Debian.

While issues #975 and #1137 have reduced the number of active mailing lists, many are still in use and require migration.

Migrating to Mailman 3 is a complex process and needs to be tested carefully. See the official migration guide for details.

Migration will also fix: #840 #1264