fix: plugins/stack-overflow/package.json to reduce vulnerabilities

The following vulnerabilities are fixed with an upgrade:
- https://snyk.io/vuln/SNYK-JS-DIFF-14917201
- https://snyk.io/vuln/SNYK-JS-BACKSTAGEBACKENDPLUGINAPI-15054291

Author: snyk-io[bot]

plugins/stack-overflow/package.json

@@ -28,7 +28,7 @@
     "@backstage/plugin-home-react": "workspace:^",
     "@backstage/plugin-search-common": "workspace:^",
     "@backstage/plugin-search-react": "workspace:^",
-    "@backstage/theme": "workspace:^",
+    "@backstage/theme": "0.1.1",
     "@material-ui/core": "^4.12.2",
     "@material-ui/icons": "^4.9.1",
     "@testing-library/jest-dom": "^5.10.1",