Align Linux UI packaging deps with Wails v3 GTK4 stack

The Wails v3 tray is a pure DBus StatusNotifierItem implementation and no
longer links libappindicator (a Fyne-era dependency). Drop the
libayatana-appindicator runtime and build deps, and move the rpm package
and dev-dependency docs onto the GTK4 / WebKitGTK 6.0 stack that the
default (non-gtk3) build actually links against.

.devcontainer/Dockerfile

@@ -6,7 +6,6 @@ RUN apt-get update && export DEBIAN_FRONTEND=noninteractive \
         iptables=1.8.9-2 \
         libgl1-mesa-dev=22.3.6-1+deb12u1 \
         xorg-dev=1:7.7+23 \
-        libayatana-appindicator3-dev=0.5.92-1 \
     && apt-get clean \
     && rm -rf /var/lib/apt/lists/* \
     && go install -v golang.org/x/tools/gopls@latest