Bump koa from 2.14.2 to 2.15.0

Bumps [koa](https://github.com/koajs/koa) from 2.14.2 to 2.15.0.
- [Changelog](https://github.com/koajs/koa/blob/2.15.0/History.md)
- [Commits](koajs/koa@2.14.2...2.15.0)

---
updated-dependencies:
- dependency-name: koa
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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

package.json

@@ -22,7 +22,7 @@
     "handlebars": "^4.7.8",
     "http-errors": "^2.0.0",
     "http-graceful-shutdown": "^3.1.13",
-    "koa": "^2.14.2",
+    "koa": "^2.15.0",
     "koa-bodyparser": "^4.4.1",
     "koa-handlebars": "^2.0.0",
     "twitter-api-v2": "^1.15.2"