chore(deps-dev): bump koa from 2.5.3 to 2.6.2

Bumps [koa](https://github.com/koajs/koa) from 2.5.3 to 2.6.2.
- [Release notes](https://github.com/koajs/koa/releases)
- [Changelog](https://github.com/koajs/koa/blob/master/History.md)
- [Commits](koajs/koa@2.5.3...2.6.2)

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