Fix: Bump the default version of jjb to 6.0.0

I highly doubt any project still works
with the old 2.8.0 value.

Issue: RELENG-5184
Change-Id: I1bb33a87349e5e7cdad024c84eb45d67b3e6a6a5
Signed-off-by: Vratko Polak <vrpolak@cisco.com>

jenkins-init-scripts/lf-env.sh

@@ -218,7 +218,7 @@ lf-activate-venv () {
         for arg in "$@"; do
             case $arg in
                 jenkins-job-builder)
-                    pkg_list+="jenkins-job-builder==${JJB_VERSION:-2.8.0} " ;;
+                    pkg_list+="jenkins-job-builder==${JJB_VERSION:-6.0.0} " ;;
                 *)                   pkg_list+="$arg " ;;
             esac
         done