Bump rubocop-performance from 1.20.2 to 1.21.0 (#1204)

Bumps [rubocop-performance](https://github.com/rubocop/rubocop-performance) from 1.20.2 to 1.21.0.
- [Release notes](https://github.com/rubocop/rubocop-performance/releases)
- [Changelog](https://github.com/rubocop/rubocop-performance/blob/master/CHANGELOG.md)
- [Commits](rubocop/rubocop-performance@v1.20.2...v1.21.0)

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

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

Gemfile.lock

@@ -511,9 +511,9 @@ GEM
       rubocop-rails (= 2.25.0)
       rubocop-rake (= 0.6.0)
       rubocop-rspec (= 2.29.2)
-    rubocop-performance (1.20.2)
+    rubocop-performance (1.21.0)
       rubocop (>= 1.48.1, < 2.0)
-      rubocop-ast (>= 1.30.0, < 2.0)
+      rubocop-ast (>= 1.31.1, < 2.0)
     rubocop-rails (2.25.0)
       activesupport (>= 4.2.0)
       rack (>= 1.1)