Bump rubocop-rails from 2.8.1 to 2.9.1 (#15390)

Bumps [rubocop-rails](https://github.com/rubocop-hq/rubocop-rails) from 2.8.1 to 2.9.1.
- [Release notes](https://github.com/rubocop-hq/rubocop-rails/releases)
- [Changelog](https://github.com/rubocop-hq/rubocop-rails/blob/master/CHANGELOG.md)
- [Commits](https://github.com/rubocop-hq/rubocop-rails/compare/v2.8.1...v2.9.1)

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

Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Gemfile.lock

@@ -546,10 +546,10 @@ GEM
       unicode-display_width (>= 1.4.0, < 2.0)
     rubocop-ast (1.3.0)
       parser (>= 2.7.1.5)
-    rubocop-rails (2.8.1)
+    rubocop-rails (2.9.1)
       activesupport (>= 4.2.0)
       rack (>= 1.1)
-      rubocop (>= 0.87.0)
+      rubocop (>= 0.90.0, < 2.0)
     ruby-progressbar (1.10.1)
     ruby-saml (1.11.0)
       nokogiri (>= 1.5.10)
@@ -784,7 +784,7 @@ DEPENDENCIES
   rspec-sidekiq (~> 3.1)
   rspec_junit_formatter (~> 0.4)
   rubocop (~> 1.7)
-  rubocop-rails (~> 2.8)
+  rubocop-rails (~> 2.9)
   ruby-progressbar (~> 1.10)
   sanitize (~> 5.2)
   scenic (~> 1.5)