Update the gen_istio_image_list script to reflect updates (#21760)
Update the gen_istio_image_list script to reflect updates (#21760)

Made changes to the script file so that it reflects the changes to the DOCKER_TARGETS
target from `DOCKER_TARGETS:=` to `DOCKER_TARGETS ?= ` in 1.5.

Fixes: #21356
istio/istio,smawson/istio,smawson/istio,smawson/istio,istio/istio,istio/istio,istio/istio,smawson/istio,smawson/istio,istio/istio,smawson/istio