Support Elixir 1.18 (#120)

Updates the `.tool-versions` file to use Elixir 1.18 for dev and also
updates the build matrix to test against Elixir 1.18 on the CI.

Author: crbelaus

.github/workflows/elixir.yml

@@ -18,12 +18,14 @@ jobs:
       fail-fast: false
       matrix:
         include:
-          - elixir: 1.15.0
-            erlang: 24.0.0
-          - elixir: 1.16.0
-            erlang: 24.0.0
-          - elixir: 1.17.0
-            erlang: 27.0.0
+          - elixir: "1.15.x"
+            erlang: "24.x"
+          - elixir: "1.16.x"
+            erlang: "24.x"
+          - elixir: "1.17.x"
+            erlang: "27.x"
+          - elixir: "1.18.x"
+            erlang: "27.x"
     services:
       db:
         image: postgres:15

.tool-versions

@@ -1,2 +1,2 @@
-elixir 1.17
+elixir 1.18
 erlang 27.0