/etc/dovecot.conf has moved to /etc/dovecot/dovecot.conf and other parts of Dovecot configuration have moved to /etc/dovecot/conf.d/*.conf. The majority of the configuration is the same and is compatible with this new version; however, you can test your configuration and list which options have been renamed, removed, or otherwise changed in this new version with the following command:
doveconf [-n] -c /old/dovecot.conf