vagrant: bump development VM images for clang/llvm

Pull in packer-ci upgrades for clang/llvm so it can run in CI:

  cilium/packer-ci-build#200

For now pull in only the net-next server box since our CI seems to
have massive problems with disk space on nodes. This is currently
being analyzed. We can bump SERVER_VERSION at a later point in time.

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

vagrant_box_defaults.rb

@@ -5,6 +5,6 @@
 $SERVER_BOX = "cilium/ubuntu-dev"
 $SERVER_VERSION= "167"
 $NETNEXT_SERVER_BOX= "cilium/ubuntu-next"
-$NETNEXT_SERVER_VERSION= "45"
+$NETNEXT_SERVER_VERSION= "48"
 @v419_SERVER_BOX= "cilium/ubuntu-4-19"
 @v419_SERVER_VERSION= "8"