Bump Elixir version in CI and remove use of slim image

Slim image causes issues with certificates, such as this error:

    ** (EXIT from #PID<0.95.0>) an exception was raised:
    ** (CaseClauseError) no case clause matching: :undefined
        (public_key 1.16.1) pubkey_os_cacerts.erl:40: :pubkey_os_cacerts.get/0
        (mix 1.17.2) lib/mix/utils.ex:664: Mix.Utils.read_httpc/1
        (mix 1.17.2) lib/mix/utils.ex:576: anonymous fn/2 in Mix.Utils.read_path/2
        (elixir 1.17.2) lib/task/supervised.ex:101: Task.Supervised.invoke_mfa/2
        (elixir 1.17.2) lib/task/supervised.ex:36: Task.Supervised.reply/4

Author: adam12

.github/workflows/main.yml

@@ -34,7 +34,7 @@ jobs:
   ecto32:
     runs-on: ubuntu-latest
     container:
-      image: elixir:1.15.7-slim
+      image: elixir:1.17.2
     services:
       postgres:
         image: postgres:14