My Jetpack: Implement Free price for Boost product

[retrofox]

This PR implements Free price for the Boost product.

Fixes #

Changes proposed in this Pull Request:

• My Jetpack: Implement Free price for Boost product

Jetpack product discussion

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

No

Testing instructions:

• Go to My Jetpack / Boost interstitial
  https://<your-testing-site>/wp-admin/admin.php?page=my-jetpack#/add-boost
  

• Confirm visually you see the Free text there

Using Storybook

• Build My Jetpack

> jetpack build packages/my-jetpack

• Run storybook

cd ./jetpack/projects/js-packages/storybook

> pnpm install & pnpm storybook:dev

Backup	Boost

[github-actions]

Thank you for your PR!

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