Bump slimmer from 18.6.2 to 18.7.0

Bumps [slimmer](https://github.com/alphagov/slimmer) from 18.6.2 to 18.7.0.
- [Changelog](https://github.com/alphagov/slimmer/blob/main/CHANGELOG.md)
- [Commits](alphagov/slimmer@v18.6.2...v18.7.0)

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

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

Gemfile.lock

@@ -270,7 +270,7 @@ GEM
     irb (1.14.3)
       rdoc (>= 4.0.0)
       reline (>= 0.4.2)
-    json (2.9.0)
+    json (2.9.1)
     json-schema (4.3.0)
       addressable (>= 2.8)
     jsonpath (1.1.5)
@@ -321,7 +321,7 @@ GEM
       net-protocol
     netrc (0.11.0)
     nio4r (2.7.4)
-    nokogiri (1.17.2)
+    nokogiri (1.18.1)
       mini_portile2 (~> 2.8.2)
       racc (~> 1.4)
     null_logger (0.0.1)
@@ -735,7 +735,7 @@ GEM
     simplecov-rcov (0.3.7)
       simplecov (>= 0.4.1)
     simplecov_json_formatter (0.1.4)
-    slimmer (18.6.2)
+    slimmer (18.7.0)
       json
       nokogiri (~> 1.7)
       null_logger