Update rubocop requirement from ~> 0.51.0 to ~> 0.81.0

Updates the requirements on [rubocop](https://github.com/rubocop-hq/rubocop) to permit the latest version.
- [Release notes](https://github.com/rubocop-hq/rubocop/releases)
- [Changelog](https://github.com/rubocop-hq/rubocop/blob/master/CHANGELOG.md)
- [Commits](rubocop/rubocop@v0.51.0...v0.81.0)

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

Author: dependabot-preview[bot]

sensu-plugins-elasticsearch.gemspec

@@ -48,7 +48,7 @@ Gem::Specification.new do |s| # rubocop:disable Metrics/BlockLength
   s.add_development_dependency 'rake',                      '~> 13.0'
   s.add_development_dependency 'redcarpet',                 '~> 3.2'
   s.add_development_dependency 'rspec',                     '~> 3.1'
-  s.add_development_dependency 'rubocop',                   '~> 0.51.0'
+  s.add_development_dependency 'rubocop',                   '~> 0.81.0'
   # 0.17 requires ruby 2.3+
   s.add_development_dependency 'test-kitchen',              '~> 1.16.0'
   s.add_development_dependency 'yard',                      '~> 0.9.11'