Add wp-downgrade to incompatible plugins in wpcomsh

[zdenys]

Adding WP Downgrade (wp-downgrade) to incompatible plugins list in wpcomsh.

Fixes https://github.com/Automattic/wpcomsh/issues/1920

Proposed changes:

• Add wp-downgrade (aka WP Downgrade) to the list of incompatible plugins since it's mentioned publicly https://wordpress.com/support/plugins/incompatible-plugins/#database-file-system-altering-plugins

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

I discovered this plugin was active on a p1725456316432119-slack-CEYCDRUL9.

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

No

Testing instructions:

• Follow the steps to test this branch (should be a comment posted by the bot below)
• On your WoA dev site, install wp-downgrade by running wp plugin install wp-downgrade --activate
• Navigate to the WoA dev site's /wp-admin/plugins.php page and confirm that the WP Downgrade is disabled.

[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, ...).
  
• ✅ Add testing instructions.
  
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• ✅ Add changelog entries to affected projects
  

This comment will be updated as you work on your PR and make changes. If you think that some of those checks are not needed for your PR, please explain why you think so. Thanks for cooperation 🤖

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Follow this PR Review Process:

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2. Choose a review path based on your changes:
   • A. Team Review: add the "[Status] Needs Team Review" label
     • For most changes, including minor cross-team impacts.
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Wpcomsh plugin:

• Next scheduled release: Atomic deploys happen twice daily on weekdays (p9o2xV-2EN-p2).

If you have any questions about the release process, please ask in the #jetpack-releases channel on Slack.

[villanovachile]

Tested and works as expected:

[villanovachile]

Next you'll want to make the necessary changes to wp-calypso as well to this file: https://github.com/Automattic/wp-calypso/blob/ff7cc132fb5d0ca61f45cffa1e22fd445035709d/client/my-sites/plugins/plugin-compatibility.js#L11