fix: plugins/events-node/package.json to reduce vulnerabilities

The following vulnerabilities are fixed with an upgrade:
- https://snyk.io/vuln/SNYK-JS-AJV-15274295

Author: snyk-io[bot]

plugins/events-node/package.json

@@ -36,7 +36,7 @@
     "postpack": "backstage-cli package postpack"
   },
   "dependencies": {
-    "@backstage/backend-plugin-api": "workspace:^"
+    "@backstage/backend-plugin-api": "0.6.19"
   },
   "devDependencies": {
     "@backstage/cli": "workspace:^"