Bump rack from 2.2.6.4 to 2.2.8.1 in /examples/heroku_api (#416)

Bumps [rack](https://github.com/rack/rack) from 2.2.6.4 to 2.2.8.1.
- [Release notes](https://github.com/rack/rack/releases)
- [Changelog](https://github.com/rack/rack/blob/main/CHANGELOG.md)
- [Commits](rack/rack@v2.2.6.4...v2.2.8.1)

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

Signed-off-by: dependabot[bot] <support@github.com>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

examples/heroku_api/Gemfile.lock

@@ -3,7 +3,7 @@ PATH
   specs:
     committee (5.0.0.beta1)
       json_schema (~> 0.14, >= 0.14.3)
-      openapi_parser (~> 1.0)
+      openapi_parser (~> 2.0)
       rack (>= 1.5)
 
 GEM
@@ -12,8 +12,8 @@ GEM
     json_schema (0.21.0)
     mustermann (2.0.2)
       ruby2_keywords (~> 0.0.1)
-    openapi_parser (1.0.0)
-    rack (2.2.6.4)
+    openapi_parser (2.0.0)
+    rack (2.2.8.1)
     rack-protection (2.2.3)
       rack
     ruby2_keywords (0.0.5)