Fix Arch Linux dependency installation (gstreamermm, webkit2gtk, -Syy) (#14171)

Fix Arch Linux dependency installation

The arch dependency script listed packages that are no longer available
in current Arch/CachyOS repositories:

- gstreamermm: removed from official repos (AUR only) and not referenced
  anywhere in the OrcaSlicer build; the build uses plain gstreamer.
- webkit2gtk: replaced upstream by webkit2gtk-4.1.

Also switch the install command from `pacman -Syy` to `pacman -Syu` to
avoid the partial-upgrade pattern that Arch officially discourages on a
rolling-release distro.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

scripts/linux.d/arch

@@ -14,7 +14,6 @@ export REQUIRED_DEV_PACKAGES=(
     glew
     gst-plugins-good
     gstreamer
-    gstreamermm
     gtk3
     libmspack
     libsecret
@@ -24,7 +23,7 @@ export REQUIRED_DEV_PACKAGES=(
     openssl
     texinfo
     wayland-protocols
-    webkit2gtk
+    webkit2gtk-4.1
     wget
 )
 
@@ -37,7 +36,7 @@ then
     done
 
     if [[ "${#NEEDED_PKGS[*]}" -gt 0 ]]; then
-        sudo pacman -Syy --noconfirm "${NEEDED_PKGS[@]}"
+        sudo pacman -Syu --noconfirm "${NEEDED_PKGS[@]}"
     fi
     echo -e "done\n"
     exit 0