Hello,
The new version is available on the official repos : https://github.com/YunoHost-Apps/synapse_ynh
I have solved the upgrade from the old version so everybody can upgrade from the old version to the new.
The upgrade could take a long time because the upgrade script migrate all data from sqlite to postgresql so it could be really long if you have a big instance with big chattroom. It could be also long for ARM arch.
I advise to upgrade by command line instead of web interface.