Investigate package description markup conversion

### Problem

While reviewing https://github.com/elementary/appcenter/pull/2099 , I noticed that we use the markup conversion routines in AppStream to convert app descriptions to plain text (albeit pre-formatted with bullet point characters etc):

https://github.com/elementary/appcenter/blob/6e78d5d0b990f05cff27a2548080099defe542c5/src/Core/Package.vala#L655-L680

Given the package descriptions are already formatted with the limited set of XML markup that AppStream supports, and in theory, the text display widgets in GTK support displaying that markup as-is, we might get some better visual results (and possibly accessibility) if we don't pre-process the markup.

### Proposal

Investigate where we use the output from a package's `get_description` and see if the widget would support displaying XML markup natively (and look better) rather than the plain-textified markup created by `markup_convert_simple`

Additionally look if `markup_convert_simple` is used anywhere else and whether it should be.

### Prior Art (Optional)

_No response_