Bump slimmer from 15.5.1 to 15.6.1

Bumps [slimmer](https://github.com/alphagov/slimmer) from 15.5.1 to 15.6.1.
- [Release notes](https://github.com/alphagov/slimmer/releases)
- [Changelog](https://github.com/alphagov/slimmer/blob/master/CHANGELOG.md)
- [Commits](alphagov/slimmer@v15.5.1...v15.6.1)

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

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

Gemfile.lock

@@ -56,7 +56,7 @@ GEM
       minitest (~> 5.1)
       tzinfo (~> 1.1)
       zeitwerk (~> 2.2, >= 2.2.2)
-    addressable (2.7.0)
+    addressable (2.8.0)
       public_suffix (>= 2.0.2, < 5.0)
     anemone (0.7.2)
       nokogiri (>= 1.3.0)
@@ -113,7 +113,7 @@ GEM
     faraday-net_http (1.0.1)
     faraday-net_http_persistent (1.1.0)
     ffi (1.13.1)
-    gds-api-adapters (71.6.0)
+    gds-api-adapters (71.8.0)
       addressable
       link_header
       null_logger
@@ -194,7 +194,7 @@ GEM
     method_source (1.0.0)
     mime-types (3.3.1)
       mime-types-data (~> 3.2015)
-    mime-types-data (3.2021.0225)
+    mime-types-data (3.2021.0704)
     mini_magick (4.10.1)
     mini_mime (1.1.0)
     mini_portile2 (2.5.3)
@@ -325,7 +325,7 @@ GEM
       docile (~> 1.1)
       simplecov-html (~> 0.11)
     simplecov-html (0.12.3)
-    slimmer (15.5.1)
+    slimmer (15.6.1)
       activesupport
       json
       nokogiri (~> 1.7)