Raise the minimum Elixir tested version to 1.14.0 (processone/ejabberd#4281)

Cannot test with Elixir 1.13.4 because its container image includes
Erlang/OTP 24.3 that ejabberd does not support anymore.

Author: badlop

.github/workflows/ci.yml

@@ -21,7 +21,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        elixir: ['1.13', '1.14', '1.15', '1.16', '1.17', '1.18']
+        elixir: ['1.14', '1.15', '1.16', '1.17', '1.18']
     runs-on: ubuntu-24.04
     container:
       image: public.ecr.aws/docker/library/elixir:${{ matrix.elixir }}