Form: Remove old block editor back-compat code and improve code quality

[talldan]

Follows up on code review comments from @ntsekouras in #41274

Again, this is a fairly big diff, but most of it is moving code 😅

Proposed changes:

• Remove old back compat code from the form block (e.g. checking whether APIs like getDefaultBlockVariation exist before using them). These APIs have been in the WordPress codebase for many versions now.
• Switch to using imported store names (import { store as blockEditorStore } from '@wordpress/block-editor';) instead of hard coded store names 'core/block-editor'.
• Extract the variation placeholder code into a separate component
• Use the useInnerBlocksProps hook instead of the InnerBlocks components. This removes one of the <div> elements from the editor markup.

Other information:

• Have you written new tests for your changes, if applicable?
• Have you checked the E2E test CI results, and verified that your changes do not break them?
• Have you tested your changes on WordPress.com, if applicable (if so, you'll see a generated comment below with a script to run)?

Does this pull request change what data or activity we track or use?

No

Testing instructions:

1. Add a 'Form' block
2. Check that all the placeholder options work - selecting different form variations and inserting patterns
3. Insert 'Contact Form' or other variations directly and check that still works.

[github-actions]

Are you an Automattician? Please test your changes on all WordPress.com environments to help mitigate accidental explosions.

• To test on WoA, go to the Plugins menu on a WordPress.com Simple site. Click on the "Upload" button and follow the upgrade flow to be able to upload, install, and activate the Jetpack Beta plugin. Once the plugin is active, go to Jetpack > Jetpack Beta, select your plugin, and enable the update/contact-form-block-code-quality branch.

• To test on Simple, run the following command on your sandbox:

  bin/jetpack-downloader test jetpack update/contact-form-block-code-quality
  

Interested in more tips and information?

• In your local development environment, use the jetpack rsync command to sync your changes to a WoA dev blog.
• Read more about our development workflow here: PCYsg-eg0-p2
• Figure out when your changes will be shipped to customers here: PCYsg-eg5-p2

[github-actions]

Thank you for your PR!

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[jeherve]

Tests well for me. 🚢

[ntsekouras]

Thank you Dan!