Bump koa from 2.13.4 to 2.15.4

Bumps [koa](https://github.com/koajs/koa) from 2.13.4 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@2.13.4...2.15.4)

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

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

Author: dependabot[bot]

package-lock.json

@@ -61,7 +61,7 @@
     }
   },
   "dependencies": {
-    "koa": "^2.13.0",
+    "koa": "^2.15.4",
     "koa-conditional-get": "^3.0.0",
     "koa-etag": "^4.0.0",
     "koa-send": "^5.0.0",