Comment 35 for bug 996916

Also ran into this issue when attempting to upgrade from 19.10 to 20.04.

After removing postgresql-10-postgis-2.4, I ran do-release-upgrade again and got this message:
   Please install all available updates for your release before upgrading.

I ran apt upgrade. It's now installing a lot of focal packages:

...snip...
832 upgraded, 116 newly installed, 0 to remove and 13 not upgraded.
Need to get 701 MB of archives.
After this operation, 1,176 MB of additional disk space will be used.
Do you want to continue? [Y/n] Y
...snip...