Configure Dependabot to update all RubyGems

By default Dependabot only monitors direct dependencies (those specified
in the Gemfile) for updates. It appears to use `bundle update
<dependency>` under the hood, which means that a Dependabot PR to update
<dependency> can also include updates to sub-dependencies. This makes it
harder to review because the PR will only include the Changelog of
<dependency> and not of the sub-dependencies.

My hope is that by configuring Dependabot to monitor direct and indirect
(sub) dependencies[1] we'll end up with more, smaller/easier to review,
PRs.

I think the same change[2] has been working reasonably well in Signon and
thought it was worth bringing authenticating-proxy into line.

[1]: https://docs.github.com/en/code-security/dependabot/dependabot-version-updates/configuration-options-for-the-dependabot.yml-file#allow
[2]: alphagov/signon#2382

.github/dependabot.yml

@@ -4,3 +4,5 @@ updates:
     directory: /
     schedule:
       interval: daily
+    allow:
+      - dependency-type: "all"