fix: package.json & yarn.lock to reduce vulnerabilities

The following vulnerabilities are fixed with an upgrade:
- https://snyk.io/vuln/SNYK-JS-KOA-8720152

package.json

@@ -33,7 +33,7 @@
     "ldap"
   ],
   "dependencies": {
-    "koa": "^1.0.0",
+    "koa": "^1.7.1",
     "koa-buddy": "^0.1.0",
     "koa-router": "^5.1.2",
     "koa-swig": "^2.0.0",

yarn.lock

@@ -292,10 +292,10 @@ koa-swig@^2.0.0:
     thenify "^3.1.0"
     utils-merge "^1.0.0"
 
-koa@^1.0.0:
-  version "1.7.0"
-  resolved "https://registry.yarnpkg.com/koa/-/koa-1.7.0.tgz#bd9c4239704b51f5dc6833058a900d79afd8901d"
-  integrity sha512-bgKsbYjJac0E8O6ya+m6KosXXUigJ15N4XFCnCA0P/kNViu9OnMLv5WcnEeQ5q1SeuKqlqcf0WiroZQBiPHp8Q==
+koa@^1.7.1:
+  version "1.7.1"
+  resolved "https://registry.yarnpkg.com/koa/-/koa-1.7.1.tgz#ccdec4160a0a1d9a867d533bafed431fae013b45"
+  integrity sha512-OWunovXTG5xliG4iVQJ2YAGa7tSDHvJiRvjEEy5WMBpkFw/O54WneFB+efsCxJoT3pRdNapSZSMvvs4bUzbkIg==
   dependencies:
     accepts "^1.2.2"
     co "^4.4.0"