Bump slimmer from 18.1.0 to 18.2.0

Bumps [slimmer](https://github.com/alphagov/slimmer) from 18.1.0 to 18.2.0.
- [Changelog](https://github.com/alphagov/slimmer/blob/main/CHANGELOG.md)
- [Commits](alphagov/slimmer@v18.1.0...v18.2.0)

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

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

Gemfile.lock

@@ -188,7 +188,7 @@ GEM
     method_source (1.0.0)
     mime-types (3.5.1)
       mime-types-data (~> 3.2015)
-    mime-types-data (3.2023.0808)
+    mime-types-data (3.2023.1003)
     mini_mime (1.1.5)
     mini_portile2 (2.8.5)
     minitest (5.20.0)
@@ -568,7 +568,7 @@ GEM
       simplecov_json_formatter (~> 0.1)
     simplecov-html (0.12.3)
     simplecov_json_formatter (0.1.4)
-    slimmer (18.1.0)
+    slimmer (18.2.0)
       json
       nokogiri (~> 1.7)
       null_logger