Remove core-js dependency

This is not used directly by the client any more, although it is a
still a transitive dependency of fetch-mock.

package.json

@@ -45,7 +45,6 @@
     "classnames": "^2.2.4",
     "codecov": "^3.1.0",
     "commander": "^9.0.0",
-    "core-js": "^3.4.1",
     "cross-env": "^7.0.0",
     "diff": "^5.0.0",
     "dompurify": "^2.0.1",

yarn.lock

@@ -2710,7 +2710,7 @@ core-js-pure@^3.0.0:
   resolved "https://registry.yarnpkg.com/core-js-pure/-/core-js-pure-3.6.5.tgz#c79e75f5e38dbc85a662d91eea52b8256d53b813"
   integrity sha512-lacdXOimsiD0QyNf9BC/mxivNJ/ybBGJXQFKzRekp1WTHoVUWsUHEn+2T8GJAzzIhyOuXA+gOxCVN3l+5PLPUA==
 
-core-js@^3.0.0, core-js@^3.4.1:
+core-js@^3.0.0:
   version "3.25.0"
   resolved "https://registry.yarnpkg.com/core-js/-/core-js-3.25.0.tgz#be71d9e0dd648ffd70c44a7ec2319d039357eceb"
   integrity sha512-CVU1xvJEfJGhyCpBrzzzU1kjCfgsGUxhEvwUV2e/cOedYWHdmluamx+knDnmhqALddMG16fZvIqvs9aijsHHaA==