Minimize CI matrix to earliest and latest versions

- Ruby: 3.2 and 4.0 (was 3.2, 3.3, 3.4, 4.0)
- Rails: 7.2 and 8.1 (was 7.2, 8.0, 8.1)
- Add RAILS_VERSION env var to dynamically set Rails version
- Update actions/checkout from v3 to v6

Reduces CI runs from 12 to 4 while still testing boundary versions.

Author: teoljungberg

.github/workflows/ci.yml

@@ -14,8 +14,8 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        ruby: ["3.2", "3.3", "3.4", "4.0"]
-        rails: ["7.2", "8.0", "8.1"]
+        ruby: ["3.2", "4.0"]
+        rails: ["7.2", "8.1"]
 
     services:
       postgres:
@@ -34,9 +34,10 @@ jobs:
 
     env:
       POSTGRES_USER: postgres
+      RAILS_VERSION: ${{ matrix.rails }}
 
     steps:
-      - uses: actions/checkout@v3
+      - uses: actions/checkout@v6
 
       - name: Set up Ruby
         uses: ruby/setup-ruby@v1

Gemfile

@@ -1,7 +1,12 @@
 source "https://rubygems.org"
 
+rails_version = ENV.fetch("RAILS_VERSION", "8.1")
+
 gemspec
 
+gem "activerecord", "~> #{rails_version}.0"
+gem "railties", "~> #{rails_version}.0"
+
 gem "bundler", ">= 1.5"
 gem "pg"
 gem "pry"