Bump qs from 2.3.3 to 6.9.3

Bumps [qs](https://github.com/ljharb/qs) from 2.3.3 to 6.9.3.
- [Release notes](https://github.com/ljharb/qs/releases)
- [Changelog](https://github.com/ljharb/qs/blob/master/CHANGELOG.md)
- [Commits](ljharb/qs@v2.3.3...v6.9.3)

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

package.json

@@ -12,7 +12,7 @@
   "repository": "koajs/qs",
   "dependencies": {
     "merge-descriptors": "~0.0.2",
-    "qs": "~2.3.3"
+    "qs": "~6.9.3"
   },
   "devDependencies": {
     "istanbul-harmony": "*",