updates-page: Improve the "updates loading" page style

This better implement the HdyStatusPage style for the updates loading state, splitting the page's title and its description in two strings, using the title case for the title and making the description a proper sentence (matching the recommendations @bertob made in the previous MR).

This also drops gs_updates_page_get_state_string() as it's only used to get a localized static string, making the code simpler and more consistent.

Before

After

A HdyStatusPage for comparison