Bump koa from 2.15.3 to 2.16.1 in /src/Umbraco.Web.UI.Client

Bumps [koa](https://github.com/koajs/koa) from 2.15.3 to 2.16.1.
- [Release notes](https://github.com/koajs/koa/releases)
- [Changelog](https://github.com/koajs/koa/blob/master/History.md)
- [Commits](koajs/koa@2.15.3...v2.16.1)

---
updated-dependencies:
- dependency-name: koa
  dependency-version: 2.16.1
  dependency-type: indirect
...

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

Author: dependabot[bot]