Bump govuk_app_config from 4.8.0 to 4.9.0

Bumps [govuk_app_config](https://github.com/alphagov/govuk_app_config) from 4.8.0 to 4.9.0.
- [Release notes](https://github.com/alphagov/govuk_app_config/releases)
- [Changelog](https://github.com/alphagov/govuk_app_config/blob/main/CHANGELOG.md)
- [Commits](alphagov/govuk_app_config@v4.8.0...v4.9.0)

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

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

Gemfile.lock

@@ -123,10 +123,12 @@ GEM
     globalid (1.0.0)
       activesupport (>= 5.0)
     govuk_ab_testing (2.4.2)
-    govuk_app_config (4.8.0)
+    govuk_app_config (4.9.0)
       logstasher (~> 2.1)
+      plek (~> 4)
       prometheus_exporter (~> 2.0)
       puma (~> 5.6)
+      rack-proxy (~> 0.7)
       sentry-rails (~> 5.3)
       sentry-ruby (~> 5.3)
       statsd-ruby (~> 1.5)
@@ -234,6 +236,8 @@ GEM
       nio4r (~> 2.0)
     racc (1.6.0)
     rack (2.2.4)
+    rack-proxy (0.7.2)
+      rack
     rack-test (2.0.2)
       rack (>= 1.3)
     rack_strip_client_ip (0.0.2)