Bump koa from 2.13.4 to 2.15.4 (rhashimoto#254)

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: indirect
...

Signed-off-by: dependabot[bot] <support@github.com>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

yarn.lock

@@ -1374,13 +1374,13 @@ __metadata:
   languageName: node
   linkType: hard
 
-"cookies@npm:~0.8.0":
-  version: 0.8.0
-  resolution: "cookies@npm:0.8.0"
+"cookies@npm:~0.9.0":
+  version: 0.9.1
+  resolution: "cookies@npm:0.9.1"
   dependencies:
     depd: "npm:~2.0.0"
     keygrip: "npm:~1.1.0"
-  checksum: 5da4d72ba81c2740511751ac8ea9506e10e2366b9ad3360333581e4667fd8d063d02c5be0bef16177de3e366b8128ed2b72921e2952c79cbca084d177e529bba
+  checksum: 4816461a38d907b20f3fb7a2bc4741fe580e7a195f3e248ef7025cb3be56a07638a0f4e72553a5f535554ca30172c8a3245c63ac72c9737cec034e9a47773392
   languageName: node
   linkType: hard
 
@@ -2552,14 +2552,14 @@ __metadata:
   linkType: hard
 
 "koa@npm:^2.13.0":
-  version: 2.13.4
-  resolution: "koa@npm:2.13.4"
+  version: 2.15.4
+  resolution: "koa@npm:2.15.4"
   dependencies:
     accepts: "npm:^1.3.5"
     cache-content-type: "npm:^1.0.0"
     content-disposition: "npm:~0.5.2"
     content-type: "npm:^1.0.4"
-    cookies: "npm:~0.8.0"
+    cookies: "npm:~0.9.0"
     debug: "npm:^4.3.2"
     delegates: "npm:^1.0.0"
     depd: "npm:^2.0.0"
@@ -2578,7 +2578,7 @@ __metadata:
     statuses: "npm:^1.5.0"
     type-is: "npm:^1.6.16"
     vary: "npm:^1.1.2"
-  checksum: 5ddd5a08dcfc17f87a90fd8717a158f6f42ac8113d8328d487cbdb1cf73797e9be199e1b1c5f67c9b6e5f0daef96654fb94ed244dea5500d8197b0f81f2dc652
+  checksum: 98de77173822f0a28c0f5d1ebd261ab02f3f905d40602e51957a0c7202122647a60c5b6c59be03361dd24bf6a5685eac97af84b306914efd057751e71f93cb0f
   languageName: node
   linkType: hard