Put the postgresql major version in the image tag, not the image name

Both of the vanilla postgresql variants now publish to the same 'postgresql' image repository, with tags like 15-- and 17--. The documentdb variant goes back to publishing to the 'postgresql-documentdb' repository, which the growthbook deployments already use, with the same style of tag.

The reason for this is that the cloudnative-pg-cluster chart builds an ImageCatalog for each cluster from a map of major version to tag, keyed by the image name. If each major version lives in its own repository, each catalog only ever contains one entry, and upgrading a cluster from 15 to 17 would mean changing which image it points at. With both majors in one repository, we can upgrade a cluster just by bumping its version number and letting the operator run pg_upgrade.

The blubber variants and the CI job names keep the major version in their names, since they describe what gets built.

Bug: T432104