Use tags for different postgres versions of an image

[SaitTalhaNisanci]

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.