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

The following vulnerabilities are fixed with an upgrade:
- https://snyk.io/vuln/SNYK-JS-JSYAML-13961110

Author: snyk-bot

package.json

@@ -13,8 +13,8 @@
     "heroku-postbuild": "npm run build"
   },
   "dependencies": {
-    "js-yaml": "3.14.2",
-    "probot": "9.15.1"
+    "js-yaml": "4.1.1",
+    "probot": "13.0.0"
   },
   "devDependencies": {
     "@types/bunyan": "1.8.11",