Add comment to vaultpress.php to ensure SAST false positives are easier to flag

[soatok]

Add comment to save others time trying to write a PoC for a non-exploitable issue

Fixes #

Proposed changes:

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

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

Testing instructions:

• Go to '..'

[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:

• ✅ Include a description of your PR changes.
  
• ✅ Add testing instructions.
  
• ✅ Specify whether this PR includes any changes to data or privacy.
  
• 🔴 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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The e2e test report can be found here. Please note that it can take a few minutes after the e2e tests checks are complete for the report to be available.

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🔴 Action required: Please add missing changelog entries for the following projects: projects/plugins/vaultpress

Use the Jetpack CLI tool to generate changelog entries by running the following command: jetpack changelog add.
Guidelines: /docs/writing-a-good-changelog-entry.md

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Vaultpress plugin:

• Next scheduled release: June 4, 2024.
• Scheduled code freeze: May 27, 2024.

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

[soatok]

I'm not going to add a changelog entry for an additional comment. That's silly.