Bump rails versions in test matrix

- 5.2.8.1 is the most recent (and last) release in the 5.x line.
- 6.0.3.7 is the most recent release in the 6.0.x line.
- 6.1.7.6 is the most recent release in the 6.1.x line.
- 7.0.8 is the most recent release in the 7.0.x line.

.github/workflows/ruby.yml

@@ -22,7 +22,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        rails-version: ['5.2.5', '6.0.3.6', '6.1.3.1', '7.0.5', '7.1.2', 'main']
+        rails-version: ['5.2.8.1', '6.0.3.7', '6.1.7.6', '7.0.8', '7.1.2', 'main']
         ruby-version: ['2.7', '3.0', '3.1', '3.2']
         exclude:
           - rails-version: '5.2.5'