fix: syncing attributes to inner blocks

[dkoo]

All Submissions:

• Have you followed the Newspack Contributing guideline?
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Changes proposed in this Pull Request:

Something must have changed in a recent Gutenberg release that broke the (admittedly hacky) method we were using to sync the parent Curated List block's attributes to its inner blocks. This PR updates the method to use updateBlockAttributes as documented for this purpose instead.

Closes #62.

How to test the changes in this Pull Request:

1. On master or epic/phase-2, create a Curated List block and set to Specific Listings mode.
2. Add several listings of any type.
3. Select the Curated List container block and change its content-related attributes, especially "Show Excerpt", "Show Category", "Show Tags", "Show Featured Image", etc.
4. Observe that the individual listings do not update to reflect the selected options.
5. Check out this branch, repeat step 3, and this time confirm that the editor preview updates as expected and the listings are shown with the expected content on the front-end.

Other information:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes, as applicable?
• Have you successfully ran tests with your changes locally?

[dkoo]

Closed in favor of #64 since the bug affects master as well.