Bump koa from 2.4.1 to 2.15.4 in /examples in the npm_and_yarn group

Bumps the npm_and_yarn group in /examples with 1 update: [koa](https://github.com/koajs/koa).


Updates `koa` from 2.4.1 to 2.15.4
- [Release notes](https://github.com/koajs/koa/releases)
- [Changelog](https://github.com/koajs/koa/blob/2.15.4/History.md)
- [Commits](koajs/koa@2.4.1...2.15.4)

---
updated-dependencies:
- dependency-name: koa
  dependency-type: direct:production
  dependency-group: npm_and_yarn
...

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