fix: plugins/proxy-backend/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/proxy-backend/package.json

@@ -35,7 +35,7 @@
   "dependencies": {
     "@backstage/backend-common": "workspace:^",
     "@backstage/backend-plugin-api": "workspace:^",
-    "@backstage/config": "workspace:^",
+    "@backstage/config": "0.1.1",
     "@types/express": "^4.17.6",
     "express": "^4.17.1",
     "express-promise-router": "^4.1.0",