Use tags for different postgres versions of an image

We currently generate one image per postgres version. For example if we are supporting 3 postgres major versions then we have `exttester-11`, `exttester-12`, `exttester-13`. Instead it could be `exttester:11.8` etc. That way we have less images and we are more explicit about the major version.

When we do this, we should update our test images in community and enterprise.