Update dovecot2 to 2.2.21, and bump PORTREVISION in consumers.

While here, fix up some paths in the decode2text plugin, and add a note
to pkg-message about how to make it work. [1]

Changes:

• doveadm mailbox list (and some others) were broken in v2.2.20
• director: Fixed making backend changes when running with only a single director server.
• virtual plugin: Fixed crash when trying to open nonexistent autocreated backend mailbox.

PR: 205231 [1]
Submitted by: andrej@ebert.su