Merge pull request #2218 from alphagov/dependabot/bundler/mongo-2.21.1

Author: govuk-ci

Gemfile.lock

@@ -266,7 +266,8 @@ GEM
     mini_mime (1.1.5)
     mini_portile2 (2.8.8)
     minitest (5.25.5)
-    mongo (2.21.0)
+    mongo (2.21.1)
+      base64
       bson (>= 4.14.1, < 6.0.0)
     mongoid (9.0.6)
       activemodel (>= 5.1, < 8.1, != 7.0.0)