chore: update CI to test only modern Elixir/OTP versions

- Update test.yml matrix to only test Elixir 1.18 & 1.19
- Update test.yml matrix to only test OTP 27 & 28
- Reduce from 18 to 4 matrix combinations
- Update mix.exs minimum Elixir version to ~> 1.18
- Remove exclude section as all combinations are compatible

Author: GSMLG-BOT

.github/workflows/elixir_ci.yml

@@ -12,14 +12,8 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        elixir: ['1.14', '1.15', '1.16', '1.17', '1.18']
-        otp: ['25', '26', '27', '28']
-        exclude:
-          # Exclude incompatible combinations
-          - elixir: '1.14'
-            otp: '28'
-          - elixir: '1.15'
-            otp: '28'
+        elixir: ['1.18', '1.19']
+        otp: ['27', '28']
 
     steps:
       - name: Checkout code

.github/workflows/test.yml

@@ -8,7 +8,7 @@ defmodule Caddy.MixProject do
     [
       app: :caddy,
       version: @version,
-      elixir: "~> 1.12",
+      elixir: "~> 1.18",
       start_permanent: Mix.env() == :prod,
       name: "Caddy",
       description: "Run Caddy Reverse Proxy Server in supervisor tree",