Bump spring from 4.2.0 to 4.2.1 in /demo

Bumps [spring](https://github.com/rails/spring) from 4.2.0 to 4.2.1.
- [Release notes](https://github.com/rails/spring/releases)
- [Changelog](https://github.com/rails/spring/blob/main/CHANGELOG.md)
- [Commits](rails/spring@v4.2.0...v4.2.1)

---
updated-dependencies:
- dependency-name: spring
  dependency-type: direct:development
  update-type: version-update:semver-patch
...

Signed-off-by: dependabot[bot] <support@github.com>

demo/Gemfile.lock

@@ -302,7 +302,7 @@ GEM
       faraday (>= 0.17.5, < 3.a)
       jwt (>= 1.5, < 3.0)
       multi_json (~> 1.10)
-    spring (4.2.0)
+    spring (4.2.1)
     spring-watcher-listen (2.1.0)
       listen (>= 2.7, < 4.0)
       spring (>= 4)