Use eslint-plugin-unused-imports v3.x.x

Fix lint error. According to the docs we should be using v3.x.x:

> * Version 3.x.x is for eslint 8 with @typescript-eslint/eslint-plugin 6 - 7
> * Version 2.x.x is for eslint 8 with @typescript-eslint/eslint-plugin 5

See: https://www.npmjs.com/package/eslint-plugin-unused-imports

Author: alanorth

package-lock.json

@@ -220,7 +220,7 @@
     "eslint-plugin-lodash": "^7.4.0",
     "eslint-plugin-rxjs": "^5.0.3",
     "eslint-plugin-simple-import-sort": "^10.0.0",
-    "eslint-plugin-unused-imports": "^2.0.0",
+    "eslint-plugin-unused-imports": "^3.2.0",
     "express-static-gzip": "^2.1.7",
     "jasmine": "^3.8.0",
     "jasmine-core": "^3.8.0",