dovecot: preset-based tls config, new presets, buster compat

dovecot/defaults/main.yml

dovecot/tasks/main.yml

dovecot/templates/conf.d/10-ssl.conf.j2

dovecot/vars/tls-intermediate.yml

dovecot/vars/tls-modern.yml

dovecot/vars/tls-old.yml

dovecot/vars/tls-previous.yml