build(deps): bump core-js in /build/frontend-legacy

Bumps [core-js](https://github.com/zloirock/core-js/tree/HEAD/packages/core-js) from 3.49.0 to 3.50.0.
- [Release notes](https://github.com/zloirock/core-js/releases)
- [Changelog](https://github.com/zloirock/core-js/blob/master/CHANGELOG.md)
- [Commits](https://github.com/zloirock/core-js/commits/v3.50.0/packages/core-js)

---
updated-dependencies:
- dependency-name: core-js
  dependency-version: 3.50.0
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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Author: dependabot[bot]