[Ubuntu] PostGIS3 conflicts with GDAL

[Jakob Miksch]

Dear list,

I run Ubuntu 18.04 with UbuntuGIS unstable.
Since a few days PostGIS3 on PostgreSQL 12 does not work anmore.

When I want to install it again, I get this message:

```
  >>> sudo apt install postgresql-12 postgresql-12-postgis-3 
postgis                                 !

Reading package lists... Done
Building dependency tree
Reading state information... Done
postgresql-12 is already the newest version (12.2-2.pgdg18.04+1).
Some packages could not be installed. This may mean that you have
requested an impossible situation or if you are using the unstable
distribution that some required packages have not yet been created
or been moved out of Incoming.
The following information may help to resolve the situation:

The following packages have unmet dependencies:
  postgis : Depends: libgdal20 (>= 2.0.1) but it is not going to be 
installed
  postgresql-12-postgis-3 : Depends: libgdal20 (>= 2.0.1) but it is not 
going to be installed
E: Unable to correct problems, you have held broken packages.
```

Any hints how to fix that?

Thanks in advance and best wishes,
Jakob