fix: replace retired ubuntu-20.04 CI runner image

Jobs queued until GitHub cancelled them at the 24h limit, so CI has not
run since the image was retired. OTP 22 is not available on newer
images, so the minimum pair moves to Elixir 1.15/OTP 24. Elixir 1.20/OTP
29 is added as the newest pair and takes over the lint steps.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

Author: dkuku

.github/workflows/ci.yml

@@ -8,19 +8,22 @@ on:
 
 jobs:
   test:
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-24.04
     env:
       MIX_ENV: test
     strategy:
       fail-fast: false
       matrix:
         include:
           - pair:
-              elixir: "1.12"
-              otp: "22"
+              elixir: "1.15"
+              otp: "24"
           - pair:
               elixir: "1.18"
               otp: "27"
+          - pair:
+              elixir: "1.20"
+              otp: "29"
             lint: lint
     steps:
       - uses: actions/checkout@v4