Protect: Sync jetpack_main_network_site option

[nateweller]

Changes proposed in this Pull Request:

• Adds jetpack_main_network_site to the Protect plugin's synced options.

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?

Jetpack product discussion

peb6dq-dd-p2#comment-142

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

Testing instructions:

• Set up a JN site with only Protect running this branch
• Connect Jetpack
• Checkout the site's Jetpack debug results and verify the multisite message is not displayed.

[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.
  
• ✅ All commits were linted before commit.
  
• ✅ Add a "[Status]" label (In Progress, Needs Team Review, ...).
  
• ✅ 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 🤖

---

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.

---

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

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

---

Once your PR is ready for review, check one last time that all required checks (other than "Required review") appearing at the bottom of this PR are passing or skipped.
Then, add the "[Status] Needs Team review" label and ask someone from your team review the code.
Once you’ve done so, switch to the "[Status] Needs Review" label; someone from Jetpack Crew will then review this PR and merge it to be included in the next Jetpack release.

---

Protect plugin:

• Next scheduled release: December 6, 2022.
• Scheduled code freeze: November 28, 2022.

[dkmyta]

LGTM!