general: Remove ellipsis from some status page titles

I remembered @bertob saying in a app review from Circle that ellipsis were unnecessary in a very similar case to this one, so that's why I opened this MR. "Downloading Software Catalog" already doesn't have ellipsis anyway.

Acc. HIG:

"Use an ellipsis (…) at the end of a label if further input or confirmation is required from the user before the action can be carried out."

This guideline isn't applicable to status page titles, as they aren't something actionable like a button.

https://developer.gnome.org/hig/guidelines/writing-style.html#writing-style

Also, use header capitalization in the "Starting up" title from gs-loading-page.