Fix an issue with BlogDashboardPersonalizationService being used on the background thread

[kean]

BlogDashboardPersonalizationService has to be main thread confined because it directly triggers the updates in the UI.

To test:

• Comment out if !service.hasPreference(for: .prompts) { to make sure the service triggers the update every time
• Open the app with Xcode debugger attaches
• Verify that Xcode doesn't throw a runtime error

Regression Notes

1. Potential unintended areas of impact: My Site
2. What I did to test those areas of impact (or what existing automated tests I relied on): n/a
3. What automated tests I added (or what prevented me from doing so): n/a

PR submission checklist:

• I have completed the Regression Notes.
• I have considered adding unit tests for my changes.
• I have considered adding accessibility improvements for my changes.
• I have considered if this change warrants user-facing release notes and have added them to RELEASE-NOTES.txt if necessary.

UI Changes testing checklist:

• Portrait and landscape orientations.
• Light and dark modes.
• Fonts: Larger, smaller and bold text.
• High contrast.
• VoiceOver.
• Languages with large words or with letters/accents not frequently used in English.
• Right-to-left languages. (Even if translation isn’t complete, formatting should still respect the right-to-left layout)
• iPhone and iPad.
• Multi-tasking: Split view and Slide over. (iPad)

[wpmobilebot]

📲 You can test the changes from this Pull Request in WordPress Alpha by scanning the QR code below to install the corresponding build.

	App Name	WordPress Alpha
	Configuration	Release-Alpha
	Build Number	pr22335-19e4234
	Version	23.9
	Bundle ID	org.wordpress.alpha
	Commit	19e4234
	App Center Build	WPiOS - One-Offs #8330

Automatticians: You can use our internal self-serve MC tool to give yourself access to App Center if needed.

[wpmobilebot]

📲 You can test the changes from this Pull Request in Jetpack Alpha by scanning the QR code below to install the corresponding build.

	App Name	Jetpack Alpha
	Configuration	Release-Alpha
	Build Number	pr22335-19e4234
	Version	23.9
	Bundle ID	com.jetpack.alpha
	Commit	19e4234
	App Center Build	jetpack-installable-builds #7353

Automatticians: You can use our internal self-serve MC tool to give yourself access to App Center if needed.