chore(deps): bump sprockets from 4d372a7 to 4.2.1

Bumps [sprockets](https://github.com/rails/sprockets) from `4d372a7` to 4.2.1. This release includes the previously tagged commit.
- [Release notes](https://github.com/rails/sprockets/releases)
- [Commits](rails/sprockets@4d372a7...v4.2.1)

---
updated-dependencies:
- dependency-name: sprockets
  dependency-type: direct:production
...

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

Gemfile.lock

@@ -39,7 +39,7 @@ GEM
       fast-stemmer (~> 1.0)
       matrix (~> 0.4)
     colorator (1.1.0)
-    concurrent-ruby (1.2.2)
+    concurrent-ruby (1.2.3)
     em-websocket (0.5.3)
       eventmachine (>= 0.12.9)
       http_parser.rb (~> 0)