Bump rubocop-rspec_rails from 2.30.0 to 2.31.0

We're changing the `.rubocop.yml` file because we were getting a
warning:

```
rubocop-rspec_rails extension supports plugin, specify `plugins:
rubocop-rspec_rails` instead of `require: rubocop-rspec_rails` in
.rubocop.yml.  For more information, see
https://docs.rubocop.org/rubocop/plugin_migration_guide.html.
```

Bumps [rubocop-rspec_rails](https://github.com/rubocop/rubocop-rspec_rails) from 2.30.0 to 2.31.0.
- [Release notes](https://github.com/rubocop/rubocop-rspec_rails/releases)
- [Changelog](https://github.com/rubocop/rubocop-rspec_rails/blob/master/CHANGELOG.md)
- [Commits](https://github.com/rubocop/rubocop-rspec_rails/compare/v2.30.0...v2.31.0)

---
updated-dependencies:
- dependency-name: rubocop-rspec_rails
  dependency-version: 2.31.0
  dependency-type: direct:development
  update-type: version-update:semver-minor
...

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

.rubocop.yml

@@ -4,7 +4,6 @@ plugins:
   - rubocop-performance
   - rubocop-rails
   - rubocop-rspec
-require:
   - rubocop-rspec_rails
 
 AllCops: