Bump slimmer from 18.6.0 to 18.6.1

Bumps [slimmer](https://github.com/alphagov/slimmer) from 18.6.0 to 18.6.1.
- [Changelog](https://github.com/alphagov/slimmer/blob/main/CHANGELOG.md)
- [Commits](alphagov/slimmer@v18.6.0...v18.6.1)

---
updated-dependencies:
- dependency-name: slimmer
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

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

Gemfile.lock

@@ -615,7 +615,7 @@ GEM
       simplecov_json_formatter (~> 0.1)
     simplecov-html (0.12.3)
     simplecov_json_formatter (0.1.4)
-    slimmer (18.6.0)
+    slimmer (18.6.1)
       json
       nokogiri (~> 1.7)
       null_logger