build(deps-dev): bump koa from 2.14.0 to 2.15.3

Bumps [koa](https://github.com/koajs/koa) from 2.14.0 to 2.15.3.
- [Changelog](https://github.com/koajs/koa/blob/2.15.3/History.md)
- [Commits](koajs/koa@2.14.0...2.15.3)

---
updated-dependencies:
- dependency-name: koa
  dependency-type: direct:development
  update-type: version-update:semver-minor
...

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