mail/dovecot24: Install imap-hibernate unconditionally

In Dovecot 2.4.5, upstream updated configure to build imap-hibernate
when kqueue notify is available (commit 6859a3aaf0e1). Since kqueue is
natively supported on all FreeBSD releases, imap-hibernate now builds
unconditionally across all versions.

Remove the obsolete OSVERSION < 1500000 check from the Makefile and
uncomment the imap-hibernate binary in pkg-plist so it gets properly
packaged on FreeBSD 14.

PR: 298059
Approved by: osa, vvd (Mentors, implicit)