conductor/stacks: set images to use for Pg14 and Pg16

[vrmiguel]

New fields:

# Within the Docker image registry used (quay.io or ECR), this is the
# organization/username the images below are contained in
organization: tembo
# Image names to use for each Pg version
# Note: images for Pg15 were kept as is as to preserve full backwards compatibility
images:
  14: "standard-cnpg:pg14-f1721c2"
  15: "standard-cnpg:15.3.0-1-3d9a853"
  16: "standard-cnpg:pg16-f1721c2"

Control Plane will use the above information to set the instance's image according to the pg_version attribute in the POST /instances endpoint (defaults to Pg15)

[nhudson]

Approved, just don't forget to go back and fix the data warehouse images.