Add .tool-versions containing Ruby 3.3

Add a `.tool-versions` so that the project works out of the box with
version managers like asdf and mise.

Add a CI check that makes sure our Ruby version in use there matches
what's in `.tool-versions` so the two don't drift.

.github/workflows/ci.yaml

@@ -84,6 +84,16 @@ jobs:
         run: bundle exec standardrb
         working-directory: ./driver/riverqueue-sequel
 
+  tool_versions_check:
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Check RUBY_VERSION matches .tool-versions Ruby version
+        run: [[ "$RUBY_VERSION" == "$(cat .tool-versions | grep ruby | cut -w -f 2)" ]] || echo "CI version \$RUBY_VERSION should match .tool-versions Ruby `cat .tool-versions | grep ruby | cut -w -f 2`" && (exit 1)
+
   type_check:
     runs-on: ubuntu-latest
     timeout-minutes: 3

.tool-versions

@@ -0,0 +1 @@
+ruby 3.3