Update rack requirement from ~> 2.0 to ~> 3.1

Updates the requirements on [rack](https://github.com/rack/rack) to permit the latest version.
- [Release notes](https://github.com/rack/rack/releases)
- [Changelog](https://github.com/rack/rack/blob/main/CHANGELOG.md)
- [Commits](rack/rack@2.2.4...v3.1.7)

---
updated-dependencies:
- dependency-name: rack
  dependency-type: direct:development
  update-type: version-update:semver-major
...

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

Author: dependabot[bot]

Gemfile.lock

@@ -75,7 +75,7 @@ GEM
       pry (>= 0.13, < 0.15)
     public_suffix (5.0.0)
     racc (1.8.1)
-    rack (2.2.14)
+    rack (3.1.15)
     rack-test (2.1.0)
       rack (>= 1.3)
     rainbow (3.1.1)
@@ -198,7 +198,7 @@ DEPENDENCIES
   overcommit (~> 0.58)
   pry
   pry-byebug
-  rack (~> 2.0)
+  rack (~> 3.1)
   rack-test
   rake (~> 13.0)
   redcarpet

oauth.gemspec

@@ -83,7 +83,7 @@ Thanks, |7eter l-|. l3oling
   spec.add_development_dependency("iconv")
   spec.add_development_dependency("minitest", "~> 5.15.0")
   spec.add_development_dependency("mocha")
-  spec.add_development_dependency("rack", "~> 2.0")
+  spec.add_development_dependency("rack", "~> 3.1")
   spec.add_development_dependency("rack-test")
   spec.add_development_dependency("rake", "~> 13.0")
   spec.add_development_dependency("rest-client")