Subscriber Login: Allow to add it to the Navigation block

[pkuliga]

Part of #34884

Proposed changes:

It allows the Subscriber Login block to be added to the Navigation block by modifying the allowedBlocks of the Navigation block.

You need to use Gutenberg v17.6 or higher to see it working.

https://make.wordpress.org/core/2024/01/31/whats-new-in-gutenberg-17-6-31-january/#new-block-json-allowed-blocks

E2E are failing because of p1706803935679559-slack-CBG1CP4EN

Other information:

• Have you written new tests for your changes, if applicable?
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Jetpack product discussion

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

Testing instructions:

Make sure it's allowed to add Subscriber Login block to the Navigation block.

[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/subscriber-login-allow-in-nav branch.

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

  bin/jetpack-downloader test jetpack update/subscriber-login-allow-in-nav
  

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[github-actions]

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[simison]

Did older versions of core block have allowedBlocks? If not, would be safer to do settings?.allowedBlocks ?? []

[pkuliga]

If there is no allowedBlocks field in settings, settings.allowedBlocks will be undefined, so settings.allowedBlocks ?? [] will evaluate to [].