build(deps-dev): bump rubocop-rails from 2.20.2 to 2.21.0

Bumps [rubocop-rails](https://github.com/rubocop/rubocop-rails) from 2.20.2 to 2.21.0.
- [Release notes](https://github.com/rubocop/rubocop-rails/releases)
- [Changelog](https://github.com/rubocop/rubocop-rails/blob/master/CHANGELOG.md)
- [Commits](rubocop/rubocop-rails@v2.20.2...v2.21.0)

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

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

Gemfile.lock

@@ -244,7 +244,7 @@ GEM
     method_source (1.0.0)
     mini_mime (1.1.5)
     mini_portile2 (2.8.4)
-    minitest (5.19.0)
+    minitest (5.20.0)
     msgpack (1.6.0)
     nenv (0.3.0)
     net-imap (0.3.7)
@@ -362,7 +362,7 @@ GEM
       rspec-mocks (~> 3.12)
       rspec-support (~> 3.12)
     rspec-support (3.12.0)
-    rubocop (1.56.1)
+    rubocop (1.56.3)
       base64 (~> 0.1.1)
       json (~> 2.3)
       language_server-protocol (>= 3.17.0)
@@ -385,7 +385,7 @@ GEM
     rubocop-performance (1.19.0)
       rubocop (>= 1.7.0, < 2.0)
       rubocop-ast (>= 0.4.0)
-    rubocop-rails (2.20.2)
+    rubocop-rails (2.21.0)
       activesupport (>= 4.2.0)
       rack (>= 1.1)
       rubocop (>= 1.33.0, < 2.0)