Form block: rename newsletter sign-up variation

[jeherve]

Fixes #31989

Proposed changes:

We have multiple blocks answering to the "newsletter" keyword nowadays. Since this variation is not a full-fledged newsletter sign-up on its own, let's avoid calling it "Newsletter".

After the change

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?
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Jetpack product discussion

• pbNhbs-7sw-p2

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

• No

Testing instructions:

Note
This variation is not available on WordPress.com simple, so you will not be able to test there.

On a site that's connected to WordPress.com, go to Posts > Add New, and search for "Newsletter". The Form Block variation should not come up. It should, however, come up if you search for "Sign-up".

[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 > Jetpack and enable the update/newsletter-sign-up-variation-name branch.

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

  bin/jetpack-downloader test jetpack update/newsletter-sign-up-variation-name
  

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[github-actions]

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Jetpack plugin:

• Next scheduled release: August 1, 2023.
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[mikestottuk]

LGTM - the simplest change. Increasing usage of the block can follow (i.e. make it more obvious "Subscribe" is for Newsletter.

[mikestottuk]

LGTM