build(deps-dev): update bigdecimal requirement from ~> 3.2, >= 3.2.0 to >= 3.2.0, ~> 4.0

Updates the requirements on [bigdecimal](https://github.com/ruby/bigdecimal) to permit the latest version.
- [Release notes](https://github.com/ruby/bigdecimal/releases)
- [Changelog](https://github.com/ruby/bigdecimal/blob/master/CHANGES.md)
- [Commits](ruby/bigdecimal@v3.2.0...v4.0.1)

---
updated-dependencies:
- dependency-name: bigdecimal
  dependency-version: 4.0.1
  dependency-type: direct:development
...

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

Author: dependabot[bot]

ruby-statistics.gemspec

@@ -34,5 +34,5 @@ Gem::Specification.new do |spec|
   spec.add_development_dependency "rspec", '~> 3.13', '>= 3.10.0'
   spec.add_development_dependency 'byebug', '>= 11.1.0', '~> 12.0'
   spec.add_development_dependency 'pry', '~> 0.14', '>= 0.14.0'
-  spec.add_development_dependency 'bigdecimal', '~> 3.2', '>= 3.2.0'
+  spec.add_development_dependency 'bigdecimal', '>= 3.2.0', '~> 4.0'
 end