Update integration test Ruby version

Rails 8 is now showing up in our integration test matrix and only
supports Ruby > 3.2.

This work updates Ruby for these tests, we still run unit tests against
older Ruby versions.

.github/workflows/rails-integration-tests.yml

@@ -42,6 +42,7 @@ jobs:
           build-args: |
             RAILS_VERSION=${{ matrix.rails }}
             MAIL_NOTIFY_BRANCH=${{ github.head_ref || github.ref_name }}
+            RUBY_VERSION=3.2.6
           push: false
           tags: mail-notify-integration-rails-${{ matrix.rails }}:latest
           outputs: type=docker, dest=/tmp/rails-${{ matrix.rails }}-image.tar