feat: upgrade elixir/erlang runtimes (#35)

Upgrade runtimes to the latest versions:

* elixir to 1.15.4
* erlang to 26.0.2

Improve CI definition by using a cache that considers the runtime version.

Save disk space by using the `slim` image variant.

Co-authored-by: Joao P Dubas <joao.dubas@gmail.com>
Reviewed-on: #35

docker-compose.yml

@@ -1,6 +1,4 @@
 ---
-version: '3.7'
-
 services:
   db:
     image: 'postgres:15.3-alpine'
@@ -15,7 +13,7 @@ services:
       - 'db_data:/var/lib/postgresql/data'
     restart: unless-stopped
   app:
-    image: 'elixir:1.14.5'
+    image: 'elixir:1.15.4-slim'
     hostname: app
     depends_on:
       - db