fix: plugins/apollo-explorer/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/apollo-explorer/package.json

@@ -25,7 +25,7 @@
     "@apollo/explorer": "^2.0.0",
     "@backstage/core-components": "workspace:^",
     "@backstage/core-plugin-api": "workspace:^",
-    "@backstage/theme": "workspace:^",
+    "@backstage/theme": "0.1.1",
     "@material-ui/core": "^4.12.2",
     "@material-ui/icons": "^4.9.1",
     "react-use": "^17.2.4",