revert to ubuntu 20.04 - OTP 23 is not available on 22.04

Author: lukaszsamson

.github/workflows/ci.yml

@@ -11,7 +11,7 @@ on:
 jobs:
   mix_test:
     name: mix test (Elixir ${{matrix.elixir}} | Erlang/OTP ${{matrix.otp}})
-    runs-on: ubuntu-22.04
+    runs-on: ubuntu-20.04
     strategy:
       fail-fast: false
       matrix: