vagrant: bump server VM image for net-next updates

This build contains latest bpf-next tree on track for 5.7 and contains:

- Socket LB helpers for cgroup programs
- BPF TPROXY helper for tc ingress
- 32bit subreg tracking, so we could potentially enable v3 CPU

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

vagrant_box_defaults.rb

@@ -5,6 +5,6 @@
 $SERVER_BOX = "cilium/ubuntu-dev"
 $SERVER_VERSION= "169"
 $NETNEXT_SERVER_BOX= "cilium/ubuntu-next"
-$NETNEXT_SERVER_VERSION= "48"
+$NETNEXT_SERVER_VERSION= "50"
 @v419_SERVER_BOX= "cilium/ubuntu-4-19"
 @v419_SERVER_VERSION= "11"