fix: plugins/stackstorm/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/stackstorm/package.json

@@ -37,7 +37,7 @@
     "@backstage/core-components": "workspace:^",
     "@backstage/core-plugin-api": "workspace:^",
     "@backstage/errors": "workspace:^",
-    "@backstage/theme": "workspace:^",
+    "@backstage/theme": "0.1.1",
     "@material-ui/core": "^4.12.2",
     "@material-ui/icons": "^4.9.1",
     "@types/react": "^16.13.1 || ^17.0.0",