mail/{dovecot,dovecot-pigeonhole}: Update to latest stable releases

• Update mail/dovecot to 2.3.21.
• Update mail/dovecot-pigeonhole to 0.5.21.
• Bump PORTREVISION of dependencies.

ReleaseNotes: https://dovecot.org/mailman3/archives/list/dovecot-news@dovecot.org/thread/Y5SJWAIAVEAQ2KKSWJM7VSJUUFRMFAH5/
PR: 273946
Approved by: maintainer timeout