Bump minitest from 5.20.0 to 5.21.1 in /slack-unfurling-esa

Bumps [minitest](https://github.com/minitest/minitest) from 5.20.0 to 5.21.1.
- [Changelog](https://github.com/minitest/minitest/blob/master/History.rdoc)
- [Commits](minitest/minitest@v5.20.0...v5.21.1)

---
updated-dependencies:
- dependency-name: minitest
  dependency-type: direct:development
  update-type: version-update:semver-minor
...

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

slack-unfurling-esa/Gemfile.lock

@@ -25,7 +25,7 @@ GEM
     mime-types (3.5.1)
       mime-types-data (~> 3.2015)
     mime-types-data (3.2023.1003)
-    minitest (5.20.0)
+    minitest (5.21.1)
     minitest-stub_any_instance (1.0.3)
     multi_xml (0.6.0)
     multipart-post (2.3.0)