Bump govuk_app_config from 9.15.6 to 9.15.7

Bumps [govuk_app_config](https://github.com/alphagov/govuk_app_config) from 9.15.6 to 9.15.7.
- [Changelog](https://github.com/alphagov/govuk_app_config/blob/main/CHANGELOG.md)
- [Commits](alphagov/govuk_app_config@v9.15.6...v9.15.7)

---
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

@@ -193,7 +193,7 @@ GEM
       rake (>= 13)
     googleapis-common-protos-types (1.16.0)
       google-protobuf (>= 3.18, < 5.a)
-    govuk_app_config (9.15.6)
+    govuk_app_config (9.15.7)
       logstasher (~> 2.1)
       opentelemetry-exporter-otlp (>= 0.25, < 0.30)
       opentelemetry-instrumentation-all (>= 0.39.1, < 0.71.0)