Update rubocop requirement from ~> 1.73.2 to ~> 1.81.0

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@v1.73.2...v1.81.0)

---
updated-dependencies:
- dependency-name: rubocop
  dependency-version: 1.81.0
  dependency-type: direct:production
...

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

Author: dependabot[bot]

Gemfile

@@ -9,4 +9,4 @@ gem 'appraisal'
 gem 'minitest', '~> 5.0'
 gem 'minitest-reporters', '~> 1.7.1'
 gem 'rake', '~> 13.2'
-gem 'rubocop', '~> 1.73.2'
+gem 'rubocop', '~> 1.81.0'