Bump govuk_app_config from 9.15.0 to 9.15.1

Bumps [govuk_app_config](https://github.com/alphagov/govuk_app_config) from 9.15.0 to 9.15.1.
- [Changelog](https://github.com/alphagov/govuk_app_config/blob/main/CHANGELOG.md)
- [Commits](alphagov/govuk_app_config@v9.15.0...v9.15.1)

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

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

Author: dependabot[bot]

Gemfile.lock

@@ -126,7 +126,7 @@ GEM
       multi_json (~> 1.11)
       os (>= 0.9, < 2.0)
       signet (>= 0.16, < 2.a)
-    govuk_app_config (9.15.0)
+    govuk_app_config (9.15.1)
       logstasher (~> 2.1)
       opentelemetry-exporter-otlp (>= 0.25, < 0.30)
       opentelemetry-instrumentation-all (>= 0.39.1, < 0.69.0)
@@ -423,9 +423,8 @@ GEM
       rack (>= 3.0.0)
     rack-test (2.1.0)
       rack (>= 1.3)
-    rackup (2.1.0)
+    rackup (2.2.0)
       rack (>= 3)
-      webrick (~> 1.8)
     rails-dom-testing (2.2.0)
       activesupport (>= 5.0.0)
       minitest