Bump slimmer from 18.0.0 to 18.1.0

Bumps [slimmer](https://github.com/alphagov/slimmer) from 18.0.0 to 18.1.0.
- [Release notes](https://github.com/alphagov/slimmer/releases)
- [Changelog](https://github.com/alphagov/slimmer/blob/main/CHANGELOG.md)
- [Commits](alphagov/slimmer@v18.0.0...v18.1.0)

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

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

Gemfile.lock

@@ -172,7 +172,7 @@ GEM
       rails-i18n
       rainbow (>= 2.2.2, < 4.0)
       terminal-table (>= 1.5.1)
-    json (2.6.2)
+    json (2.6.3)
     json-schema (3.0.0)
       addressable (>= 2.8)
     kgio (2.11.4)
@@ -232,7 +232,7 @@ GEM
     puma (6.0.2)
       nio4r (~> 2.0)
     racc (1.6.2)
-    rack (2.2.5)
+    rack (2.2.6)
     rack-proxy (0.7.4)
       rack
     rack-test (2.0.2)
@@ -343,7 +343,7 @@ GEM
       simplecov_json_formatter (~> 0.1)
     simplecov-html (0.12.3)
     simplecov_json_formatter (0.1.4)
-    slimmer (18.0.0)
+    slimmer (18.1.0)
       json
       nokogiri (~> 1.7)
       null_logger