Bump rubocop-rails from 2.8.0 to 2.8.1 (mastodon#14832)

Bumps [rubocop-rails](https://github.com/rubocop-hq/rubocop-rails) from 2.8.0 to 2.8.1.
- [Release notes](https://github.com/rubocop-hq/rubocop-rails/releases)
- [Changelog](https://github.com/rubocop-hq/rubocop-rails/blob/master/CHANGELOG.md)
- [Commits](rubocop/rubocop-rails@v2.8.0...v2.8.1)

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

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

Gemfile.lock

@@ -500,7 +500,7 @@ GEM
       redis-store (>= 1.2, < 2)
     redis-store (1.9.0)
       redis (>= 4, < 5)
-    regexp_parser (1.7.1)
+    regexp_parser (1.8.0)
     request_store (1.5.0)
       rack (>= 1.4)
     responders (3.0.1)
@@ -544,9 +544,9 @@ GEM
       rubocop-ast (>= 0.3.0, < 1.0)
       ruby-progressbar (~> 1.7)
       unicode-display_width (>= 1.4.0, < 2.0)
-    rubocop-ast (0.4.0)
+    rubocop-ast (0.4.2)
       parser (>= 2.7.1.4)
-    rubocop-rails (2.8.0)
+    rubocop-rails (2.8.1)
       activesupport (>= 4.2.0)
       rack (>= 1.1)
       rubocop (>= 0.87.0)