fix: upgrade core-js to 2.6.12

[katesaikishore]

Description

This PR addresses a vulnerability in core-js@2.6.11 by upgrading it to version 2.6.12.
Since core-js is a transitive dependency (via @babel/polyfill -> auth0-extensions-cli), we have added core-js@2.6.12 as a direct dependency in package.json to force the use of the patched version.

Changes

• Modified package.json to add "core-js": "2.6.12" to dependencies.
• Updated yarn.lock to reflect the new dependency resolution.

Verification

The following tests were performed to verify the fix:

1. Dependency Check: Ran yarn why core-js to confirm that core-js is resolved to 2.6.12.
2. Build Verification: Ran npm run extension:build to ensure the extension builds successfully with the new dependency.
   • Note: For Node.js 18+, NODE_OPTIONS=--openssl-legacy-provider was used to bypass OpenSSL legacy provider issues.

Closes operational risk ticket for core-js@2.6.11.