Disable CI on Postgres version <= 11 fix change setup

Since around April 15th the tests on Postgres started failing due to
having issues will running `sudo apt-get install postgresql-<version>`.
This is done internally by the `ankane/setup-postgres` step and up to
that point worked without issues. Up to that date we used Ubuntu 18.04
but there, no Postgres version could be installed via the script
anymore. While investigating, we found that on Ubuntu 20.04 Postgres 12
and 13 could at least still be installed.

This was unrelated to any changes done in this repository. Therefore, to
unblock further development of this gem, upgrade the PG tests to use
Ubuntu 20.04 and remove the PG versions that no longer seem to work from
the test matrix.

Also, create #9 to
re-enable tests for older Postgres versions to ensure compatibility of
the gem.

.github/workflows/test.yml

@@ -46,11 +46,11 @@ jobs:
           DB_USER: root
 
   test-on-postgres:
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-20.04
     strategy:
       matrix:
         ruby-version: ['2.6', '2.7', '3.0']
-        postgres-version: [13, 12, 11, 10, 9.6]
+        postgres-version: [13, 12]
 
     steps:
       - uses: actions/checkout@v2