Update synctools #762
Description
After the synctools calendar rewrite updating in ICSx⁵ will be a substantial amount of work. We should update synctools one commit at a time (or a few, if there are very little changes) and incorporate the changes gradually - like it was done in DAVx⁵.
ICSx⁵ references commits from bitfireAT/synctools#169 until it catches up with synctools main branch.
We can add sub-issues to this one until synctools is fully up-to-date again.