Don't conflate `high` and `urgent` portal notification priorities

### What Happened?

the XDG Desktop Portals specification has 4 notification levels (unlike FreeDesktop's 3), one of them being `high`. Right now Pantheon will treat `high` the same as `urgent`, bypassing Do-Not-Disturb and doing the usual attention-grabbing shenanigans.

This is unlike any other desktop environment including GNOME and Plasma, where `high` is treated like `normal`. This specifically causes me the issue of GNOME Fractal bypassing Do-Not-Disturb despite me not wanting it to.

### Steps to Reproduce

1. Receive a Notification with `high` priority from within a flatpak / behind the portal
2. Pantheon treats it like FreeDesktop `urgent`

### Expected Behavior

`high` XDG Portals notification priority should be treated like `normal` freeDesktop priority, as to not have it unintentionally bypass DND.

### OS Version

8.x (Circe)

### Session Type

Secure Session (Wayland)

### Software Version

Latest release (I have run all updates)

### Log Output

```shell

```

### Hardware Info

_No response_

Start by tracing the XDG Desktop Portal notification-priority handling in the Vala code; the issue does not identify a file or test. Reproduce a portal notification with `high` priority, then verify that it follows `normal` behavior and does not bypass Do-Not-Disturb while `urgent` retains its existing attention-grabbing behavior.

Written by the indexing model from the issue text.