Update dependency rubocop-rails to v2.21.0 (#3061)

Co-authored-by: renovate[bot] <29139614+renovate[bot]@users.noreply.github.com>

Gemfile.lock

@@ -212,7 +212,7 @@ GEM
       rails (>= 2.3.3)
     mini_mime (1.1.5)
     mini_portile2 (2.8.4)
-    minitest (5.19.0)
+    minitest (5.20.0)
     minitest-optional_retry (0.0.2)
       minitest (~> 5.0)
     minitest-spec-rails (7.2.0)
@@ -356,7 +356,7 @@ GEM
       unicode-display_width (>= 2.4.0, < 3.0)
     rubocop-ast (1.29.0)
       parser (>= 3.2.1.0)
-    rubocop-rails (2.20.1)
+    rubocop-rails (2.21.0)
       activesupport (>= 4.2.0)
       rack (>= 1.1)
       rubocop (>= 1.33.0, < 2.0)