Bump slimmer from 18.6.1 to 18.6.2

Bumps [slimmer](https://github.com/alphagov/slimmer) from 18.6.1 to 18.6.2.
- [Changelog](https://github.com/alphagov/slimmer/blob/main/CHANGELOG.md)
- [Commits](alphagov/slimmer@v18.6.1...v18.6.2)

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

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

Gemfile.lock

@@ -224,7 +224,7 @@ GEM
     method_source (1.0.0)
     mime-types (3.5.2)
       mime-types-data (~> 3.2015)
-    mime-types-data (3.2024.0820)
+    mime-types-data (3.2024.0903)
     mini_mime (1.1.5)
     mini_portile2 (2.8.7)
     minitest (5.25.1)
@@ -236,7 +236,6 @@ GEM
     mocha (2.4.5)
       ruby2_keywords (>= 0.0.5)
     msgpack (1.7.2)
-    mutex_m (0.2.0)
     net-imap (0.4.16)
       date
       net-protocol
@@ -600,7 +599,7 @@ GEM
       simplecov_json_formatter (~> 0.1)
     simplecov-html (0.12.3)
     simplecov_json_formatter (0.1.4)
-    slimmer (18.6.1)
+    slimmer (18.6.2)
       json
       nokogiri (~> 1.7)
       null_logger