publishing-requirements: Allow use of single svg icon instead of svg icons for each size

### Problem

We explicit that third-party app developers must provide icons in size of 32px, 48px, 64px, and 128px at least in [the publishing requirements](https://docs.elementary.io/develop/appcenter/publishing-requirements#icons):

> Your app must install icons to `/usr/share/icons/hicolor` in 32px, 48px, 64px, and 128px. These are the icon sizes that will be displayed in the Applications menu's search, grid, and category views, Multitasking View, the dock, and in AppCenter.

And we also follow this requirement by ourselves; we provide svg icons in these sizes in our core apps.

However, most of GNOME apps only provide single scalable svg icon instead of svg icons for each size:

- [Web](https://gitlab.gnome.org/GNOME/epiphany/-/tree/main/data/icons/hicolor/scalable/apps?ref_type=536664ad3ab7f044d24688f0a94666ea63b5def5)
- [Papers](https://gitlab.gnome.org/GNOME/Incubator/papers/-/tree/main/data/icons/scalable/apps?ref_type=c6000bf274d1dff96c9bb0e61a2380facc4b3af0)
- [gnome-calcurator](https://gitlab.gnome.org/GNOME/gnome-calculator/-/tree/main/data/icons/hicolor/scalable/apps?ref_type=ff8923f8b874acf055b36c9d3761e18b94ba1db1)
- [gnome-sudoku](https://gitlab.gnome.org/GNOME/gnome-sudoku/-/tree/main/data/icons/hicolor/scalable?ref_type=4adec3f289766146aeb28d168684f95a1cbf0ac0)
- [meld](https://gitlab.gnome.org/GNOME/meld/-/tree/main/data/icons/hicolor/scalable/apps?ref_type=c32a27378796c9dc4351e0808d944ac66262ab85)

Flathub also does not require app icons for each size:

https://docs.flathub.org/docs/for-app-authors/requirements#icons

### Proposal

Allow use of single svg icon instead of svg icons for each size.

I think developers must still provide icons for each size if in PNG format, but they don't need to provide them if in *Scalable* Vector Graphics format, unless they want to do so for some reason e.g. detailed design in 128px.

### Prior Art (Optional)

- [Icon requrements for Flathub submission](https://docs.flathub.org/docs/for-app-authors/requirements#icons)
- [Icon conventions by Flatpak](https://docs.flatpak.org/en/latest/conventions.html#application-icons)