Bump koa from 2.14.1 to 2.15.4

Bumps [koa](https://github.com/koajs/koa) from 2.14.1 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@2.14.1...2.15.4)

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

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

packages/site-server/package.json

@@ -119,7 +119,7 @@
     "@webcomponents/internal-site-content": "^0.0.0",
     "@webcomponents/internal-site-templates": "^0.0.0",
     "google-auth-library": "^8.7.0",
-    "koa": "^2.13.4",
+    "koa": "^2.15.4",
     "koa-conditional-get": "^3.0.0",
     "koa-etag": "^4.0.0",
     "koa-static": "^5.0.0",