Bump govuk_app_config from 9.15.3 to 9.15.4

Bumps [govuk_app_config](https://github.com/alphagov/govuk_app_config) from 9.15.3 to 9.15.4.
- [Changelog](https://github.com/alphagov/govuk_app_config/blob/main/CHANGELOG.md)
- [Commits](alphagov/govuk_app_config@v9.15.3...v9.15.4)

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

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

Gemfile.lock

@@ -148,7 +148,7 @@ GEM
       rinku (~> 2.0)
       sanitize (~> 6)
     govuk_ab_testing (3.0.0)
-    govuk_app_config (9.15.3)
+    govuk_app_config (9.15.4)
       logstasher (~> 2.1)
       opentelemetry-exporter-otlp (>= 0.25, < 0.30)
       opentelemetry-instrumentation-all (>= 0.39.1, < 0.70.0)
@@ -236,7 +236,7 @@ GEM
     mime-types-data (3.2024.1105)
     mini_mime (1.1.5)
     mini_portile2 (2.8.8)
-    minitest (5.25.2)
+    minitest (5.25.4)
     minitest-reporters (1.7.1)
       ansi
       builder