Bump async from 2.20.0 to 2.21.0

Bumps [async](https://github.com/socketry/async) from 2.20.0 to 2.21.0.
- [Release notes](https://github.com/socketry/async/releases)
- [Changelog](https://github.com/socketry/async/blob/main/releases.md)
- [Commits](socketry/async@v2.20.0...v2.21.0)

---
updated-dependencies:
- dependency-name: async
  dependency-type: indirect
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <[email protected]>

Gemfile.lock

@@ -34,7 +34,7 @@ GEM
       public_suffix (>= 2.0.2, < 7.0)
     afm (0.2.2)
     ansi (1.5.0)
-    async (2.20.0)
+    async (2.21.0)
       console (~> 1.29)
       fiber-annotation
       io-event (~> 1.6, >= 1.6.5)