Bump govuk_app_config from 9.15.7 to 9.16.4

Bumps [govuk_app_config](https://github.com/alphagov/govuk_app_config) from 9.15.7 to 9.16.4.
- [Changelog](https://github.com/alphagov/govuk_app_config/blob/main/CHANGELOG.md)
- [Commits](alphagov/govuk_app_config@v9.15.7...v9.16.4)

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

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]