CI: Update PROJECT_VERSION and KAS_REPO_REF_DIR variables

The value of PROJECT_VERSION is not set, which makes the values of the
variables related to the Yocto cache (SSTATE_DIR, DL_DIR, etc) in the
source code inconsistent with the values set from the GitLab CI/CD
settings UI. Set it to vNext to make it consistent.

Also, KAS_REPO_REF_DIR is changed to the setting in templates.

Signed-off-by: Robbie Cao <robbie.cao@arm.com>