Set explicitly ubuntu to 20.04 for test

As in current `ubuntu-latest` not all python version are available we
switch explicitly back to `ubunt-20.04`.
We also remove `python 3.6` from supported python distributions because
it does not safely provide needed requests version.

Author: cmeissner

.github/workflows/main.yml

@@ -7,7 +7,7 @@ on:
 jobs:
   e2e_tests:
     name: end to end tests
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     strategy:
       matrix:
         phpipam-version: ['1.4x', '1.5x']