RAID1 is read-only after upgrade to Ubuntu 17.10

This won’t explain why your array ended up in read-only mode, but

mdadm --readwrite /dev/md0

should return it to normal. In your case it doesn’t, and the reason isn’t entirely obvious: if constituent devices are themselves read-only, the RAID array is read-only (which is what matches the behaviour you’re seeing, and the code-paths used when you try to re-enable read-write).

The dmsetup table information gives a strong hint as to what’s going on: osprober (I imagine, given the names of the devices) is finding the real RAID components, and for some reason it’s creating device mapper devices on top of them, and these are being picked up and used for the RAID device. Since the only device mapper devices are those two osprober devices, the simplest solution is to stop the RAID device, stop the DM devices, and re-scan the RAID array so that the underlying components devices are used. To stop the DM devices, run

dmsetup remove_all

as root.