feat: upgrade runtime and system dependencies (#5)

* Upgrade elixir, erlang, and postgres to their latest versions.
* Fix CI configuration, so tests are executed.
* Migrate project configuration from `Mix.Config` to `Config`.

Reviewed-on: #5

config/prod.exs

@@ -1,4 +1,4 @@
-use Mix.Config
+import Config
 
 # For production, don't forget to configure the url host
 # to something meaningful, Phoenix uses this information