The latest minor release of PostgreSQL came out today (11.5, 10.10, 9.6.15, 9.5.19, 9.4.24) and contains a fix to the ALTER TABLE statement to allow the install of Grafana to proceed without errors anymore.
Updating to the latest minor release only requires a restart and installing the package on most systems will automatically do that for you (Debian/Ubuntu/RHEL/CentOS).