Update Newspack Blocks to v1.21.0

[dkoo]

• Bumps Newspack Blocks to v1.21.0
• Includes a fix for Homepage Posts: excerpt in editor displaying extraneous content newspack-blocks#700 (fix PR: refactor: render custom excerpt length server-side newspack-blocks#701)
• Also includes a fix for PHPCS "unused variable" warnings: chore: add phpcs unused variable rule newspack-blocks#702

To test:

• In the apps/editing-toolkit folder, yarn && yarn run build to build changes and sync the new Blocks release.
• Load the changes on a sandbox and verify that Homepage Posts: excerpt in editor displaying extraneous content newspack-blocks#700 is no longer reproducible.
• Load the changes on an Atomic site and verify that the issue is no longer reproducible there, either

[matticbot]

Test live: https://calypso.live/?branch=update/etk-newspack-blocks-1.21.0

[matticbot]

This PR does not affect the size of JS and CSS bundles shipped to the user's browser.

Generated by performance advisor bot at iscalypsofastyet.com.

[matticbot]

Caution: This PR affects files in the Editing Toolkit Plugin on WordPress.com
Please ensure your changes work on WordPress.com before merging.

D57166-code has been created so you can easily test it on your sandbox. See this FieldGuide page about developing the Editing Toolkit Plugin for more info: PCYsg-ly5-p2

[creativecoder]

I can verify that this fixes Automattic/newspack-blocks#700 for the Blog Posts block
✅ On Simple Sites by applying D57166-code
✅ On a Jetpack site by running this build of the Editing Toolkit plugin

[noahtallen]

The PHPlint also works great locally, even after removing the override we had for newspack blocks.