Update sinatra requirement from ~> 2.1 to ~> 3.1

Updates the requirements on [sinatra](https://github.com/sinatra/sinatra) to permit the latest version.
- [Changelog](https://github.com/sinatra/sinatra/blob/main/CHANGELOG.md)
- [Commits](sinatra/sinatra@v2.1.0...v3.1.0)

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

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

Gemfile

@@ -12,6 +12,6 @@ gem "rake", "~> 13.0"
 gem "rspec", "~> 3.10"
 gem "rubocop", "~> 1.22"
 gem "rubocop-rake", require: false
-gem "sinatra", "~> 2.1"
+gem "sinatra", "~> 3.1"
 
 gemspec