Bump rubocop-rails from 2.20.2 to 2.21.2

Bumps [rubocop-rails](https://github.com/rubocop/rubocop-rails) from 2.20.2 to 2.21.2.
- [Release notes](https://github.com/rubocop/rubocop-rails/releases)
- [Changelog](https://github.com/rubocop/rubocop-rails/blob/master/CHANGELOG.md)
- [Commits](https://github.com/rubocop/rubocop-rails/compare/v2.20.2...v2.21.2)

---
updated-dependencies:
- dependency-name: rubocop-rails
  dependency-type: direct:development
  update-type: version-update:semver-minor
...

Note version 2.21.0 relaxes the default `Include` path for
`Rails/FindEach`, and so this version can find and correct offenses
outside the `app/models/` folder [1].

Also note this version replaces `unless something.include?` with `if
something.exclude?`; since we don't use the `exclude?` method anywhere,
we're removing the `include?` method from the list of methods checked by
this cop.

Finally, the Rails/HttpStatus method now returns a false positive when
rendering a dashboard partial and passing the `status` variable. In
order to avoid this issue, we could change the name of the local
variable or move the partial to a component, but for now we're simply
excluding these files for this cop.

[1] https://github.com/rubocop/rubocop-rails/pull/1059/commits/0066b3505

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

Gemfile.lock

@@ -536,7 +536,7 @@ GEM
     rubocop-performance (1.19.1)
       rubocop (>= 1.7.0, < 2.0)
       rubocop-ast (>= 0.4.0)
-    rubocop-rails (2.20.2)
+    rubocop-rails (2.21.2)
       activesupport (>= 4.2.0)
       rack (>= 1.1)
       rubocop (>= 1.33.0, < 2.0)
@@ -754,7 +754,7 @@ DEPENDENCIES
   rubocop-capybara (~> 2.18.0)
   rubocop-factory_bot (~> 2.24.0)
   rubocop-performance (~> 1.19.1)
-  rubocop-rails (~> 2.20.2)
+  rubocop-rails (~> 2.21.2)
   rubocop-rspec (~> 2.24.1)
   rvm1-capistrano3 (~> 1.4.0)
   sassc-rails (~> 2.1.2)