Important stuff first, Trixie is a toy Triceratops, from Toy Story movies, probably from the same set as Rex, owned by Bonnie Anderson.
Read the release notes, yeah long peice of text, but well worth it to avoid problems with transforming your worm to a triceratops dinosaur.
Edit the /etc/ssh/sshd_config to include latest algorithms:
KexAlgorithms mlkem768x25519-sha256,sntrup761x25519-sha512,curve25519-sha256,curve25519-sha256@libssh.org
Why this is not 3.0 is beyond me. Massive changes, consult the official documentation and test extensively. Manual action required.
A few years ago I have used tutorial by Thomas Leister and here is the update for Dovecot 2.4 that really helps.