snap: Enable kde-neon extension

[hebasto]

This PR is an amendment to #276, which was incomplete and caused some issues.

According to the Snapcraft documentation, the use of snapcraft-desktop-helpers, which was removed in #276, has been deprecated in favor of the kde-neon extension.

This PR addresses the resulting issue.

For Bitcoin Core v30.0 using Qt 6, the following diff should be applied:

--- a/snap/snapcraft.yaml
+++ b/snap/snapcraft.yaml
@@ -8,7 +8,7 @@ description: |
 
 grade: stable
 confinement: strict
-base: core20
+base: core22
 
 apps:
   daemon:
@@ -23,7 +23,7 @@ apps:
       HOME: $SNAP_USER_COMMON
   qt:
     command: bin/bitcoin-qt
-    extensions: [kde-neon]
+    extensions: [kde-neon-6]
     plugs: [home, removable-media, network, network-bind, desktop, x11, unity7]
     environment:
       HOME: $SNAP_USER_COMMON

It remains unclear how to correctly handle the application icon in some desktop environments or UI components, such as Ubuntu Dock.

[maflcko]

core22

pls see #293