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]>

Gemfile.lock

@@ -238,7 +238,7 @@ GEM
       jquery-rails (~> 4.3)
       plek (>= 2.1)
       rails (>= 6)
-    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)
@@ -824,7 +824,7 @@ GEM
       mongoid (>= 4.0.0)
       state_machines-activemodel (>= 0.5.0)
     statsd-ruby (1.5.0)
-    stringio (3.1.1)
+    stringio (3.1.2)
     strip_attributes (1.13.0)
       activemodel (>= 3.0, < 8.0)
     terminal-table (3.0.2)