Update steep requirement from ~> 1.7.0 to ~> 1.8.1

Updates the requirements on [steep](https://github.com/soutaro/steep) to permit the latest version.
- [Release notes](https://github.com/soutaro/steep/releases)
- [Changelog](https://github.com/soutaro/steep/blob/master/CHANGELOG.md)
- [Commits](soutaro/steep@v1.7.0...v1.8.1)

---
updated-dependencies:
- dependency-name: steep
  dependency-type: direct:development
...

Signed-off-by: dependabot[bot] <[email protected]>

Gemfile

@@ -6,7 +6,7 @@ gemspec
 group :development do
   group :type_check do
     gem 'rbs', '~> 3.6.1', require: false
-    gem 'steep', '~> 1.7.0', require: false
+    gem 'steep', '~> 1.8.1', require: false
   end
   group :documentation do
     gem 'yard', github: 'ParadoxV5/yard', tag: 'commonmarker-1.0_1', require: false