Remove mention of mobile apps and update apps image

Part of #2281 (closed)

Admittedly, the new version is more blurry than the old one, but it's more accurate/up to date. This is as far as my Gimp skills go. This MR is more of a suggesting/conversation starter, doesn't have to get merged.

Gimp file: https://gitlab.com/gitterHQ/design/-/blob/master/design-assets/deliverables/frontpage/gitter-apps.xcf

Before

After