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] <support@github.com>

Gemfile.lock

@@ -238,7 +238,7 @@ GEM
     listen (3.9.0)
       rb-fsevent (~> 0.10, >= 0.10.3)
       rb-inotify (~> 0.9, >= 0.9.10)
-    logger (1.6.3)
+    logger (1.6.4)
     logstasher (2.1.5)
       activesupport (>= 5.2)
       request_store
@@ -646,7 +646,7 @@ 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