Bump slimmer from 17.0.0 to 18.0.0

Bumps [slimmer](https://github.com/alphagov/slimmer) from 17.0.0 to 18.0.0.
- [Release notes](https://github.com/alphagov/slimmer/releases)
- [Changelog](https://github.com/alphagov/slimmer/blob/main/CHANGELOG.md)
- [Commits](alphagov/slimmer@v17.0.0...v18.0.0)

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

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

Gemfile.lock

@@ -211,7 +211,7 @@ GEM
       net-protocol
     netrc (0.11.0)
     nio4r (2.5.8)
-    nokogiri (1.13.8)
+    nokogiri (1.13.9)
       mini_portile2 (~> 2.8.0)
       racc (~> 1.4)
     null_logger (0.0.1)
@@ -339,8 +339,7 @@ GEM
       simplecov_json_formatter (~> 0.1)
     simplecov-html (0.12.3)
     simplecov_json_formatter (0.1.4)
-    slimmer (17.0.0)
-      activesupport
+    slimmer (18.0.0)
       json
       nokogiri (~> 1.7)
       null_logger