[oobe] fix scoobe page threading issues

[jaimecbernardo]

Summary of the Pull Request

What is this about:
The use of await in the "What's New" page is not returning to the UI thread as expected, even if context should be maintained as mentioned in the await documentation. This happens when running PowerToys.Settings directly when debugging from VS. This might be a XAML islands quirk and might happen in other situations, as well, so let's make sure it runs in the right thread.

What is included in the PR:
Dispatch code to the right thread after await, before doing UI element changes.

How does someone test / validate:
Just looking for that Code Quality check, but running scoobe to make sure it still works is fair game as well.

Quality Checklist

• Linked issue: Upgrade User OOBE experience (SCOOBE) #14536
• Communication: I've discussed this with core contributors in the issue.
• Tests: Added/updated and all pass
• Installer: Added/updated and all pass
• Localization: All end user facing strings can be localized
• Docs: Added/ updated
• Binaries: Any new files are added to WXS / YML
  • No new binaries
  • YML for signing for new binaries
  • WXS for installer for new binaries

[stefansjfw]

Installed PT, What's new shows and works good.
Also running Settings directly from VS does not throw any exceptions and works correctly