Boost: Prompt c.css regen when updating foundation pages list

[dilirity]

Proposed changes:

• Show a "regenerate critical css" notice when updating foundation pages list.

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)?

Jetpack product discussion

n/a

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

no

Testing instructions:

• Setup Boost (free);
• Enable critical css and generate;
• Add a few URLs to the foundation pages list;
• The notice prompting critical regen should show.

[github-actions]

Thank you for your PR!

When contributing to Jetpack, we have a few suggestions that can help us test and review your patch:

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• ✅ Add a "[Status]" label (In Progress, Needs Team Review, ...).
  
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