Update "bugs" in package.json to point to ckeditor5

[Reinmar]

We plan to merge issue trackers into one. Let's do this change in package.jsons already so it's up to date on npm.

[Reinmar]

@pomek, let's have a quick call about this.

[pomek]

After introducing proposed changes, the changelog generator will contain invalid URLs to issues in changelog entries.

https://github.com/ckeditor/ckeditor5-dev/blob/28e3b533d9667cbfda45f09a37e103f171649ff0/packages/ckeditor5-dev-env/lib/release-tools/utils/transform-commit/transform-commit-utils.js#L80

Instead of parsing pkg.bugs entry, we can take the value from pkg.repository.