fix: upgrade core-js from 2.6.11 to 2.6.12

Snyk has created this PR to upgrade core-js from 2.6.11 to 2.6.12.

See this package in npm:
https://www.npmjs.com/package/core-js

See this project in Snyk:
https://app.snyk.io/org/mailchain/project/c65c678c-017e-408f-af91-3c59616d8b00?utm_source=github&utm_medium=upgrade-pr

package.json

@@ -22,7 +22,7 @@
     "@ckeditor/ckeditor5-angular": "^1.2.3",
     "@ckeditor/ckeditor5-build-classic": "github:mailchain/ckeditor5-build-classic#mailchain_stable",
     "bootstrap": "^4.5.1",
-    "core-js": "^2.6.11",
+    "core-js": "^2.6.12",
     "ethereum-blockies-base64": "^1.0.2",
     "font-awesome": "^4.7.0",
     "glyphicons-halflings": "^1.9.1",