Update rubocop requirement from ~> 1.76.0 to ~> 1.81.1

Updates the requirements on [rubocop](https://github.com/rubocop/rubocop) to permit the latest version.
- [Release notes](https://github.com/rubocop/rubocop/releases)
- [Changelog](https://github.com/rubocop/rubocop/blob/master/CHANGELOG.md)
- [Commits](rubocop/rubocop@v1.76.0...v1.81.1)

---
updated-dependencies:
- dependency-name: rubocop
  dependency-version: 1.81.1
  dependency-type: direct:production
...

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

Author: dependabot[bot]

Gemfile

@@ -8,5 +8,5 @@ gem "bundler"
 gem "pry"
 gem "rake"
 gem "rspec", "~> 3.1"
-gem "rubocop", "~> 1.76.0"
+gem "rubocop", "~> 1.81.1"
 gem "simplecov", "!= 0.18.0", "!= 0.18.1", "!= 0.18.2", "!= 0.18.3", "!= 0.18.4", "!= 0.18.5", "!= 0.19.0", "!= 0.19.1"