Update and separate test images

The build image was a single one and it would contain pg11, pg12 and
pg13. Now it is separated so that we can build each pg major
independently.

Tags are used as full postgres versions so that we can know which
version we use by looking at the tag. For example exttester:11.9 would
mean we are using pg11.9.

pg11 is updated from 11.5 to 11.9.
pg12 is updated from 12rc to 12.4.