Bump koa from 2.7.0 to 2.15.4 in /day/blog/blog-backend

Bumps [koa](https://github.com/koajs/koa) from 2.7.0 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@2.7.0...2.15.4)

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

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