Update actions/checkout

This dependency was relying on node v16, which github are deprecating
support for.

This will resolve the issue for one of the actions listed here

```
Node.js 16 actions are deprecated. Please update the following actions
to use Node.js 20: actions/checkout@v3

For more information see: https://github.blog/changelog/2023-09-22-github-actions-transitioning-from-node-16-to-node-20/.
```

.github/workflows/spec.yaml

@@ -5,8 +5,8 @@ jobs:
  build:
  runs-on: ubuntu-latest
  steps:
- - uses: actions/checkout@v2
- - uses: ruby/setup-ruby@v1
- with:
- bundler-cache: true
- - run: bundle exec rspec
+  - uses: actions/checkout@v4
+  - uses: ruby/setup-ruby@v1
+  with:
+  bundler-cache: true
+  - run: bundle exec rspec