fixing Dockerhub link(s) to the Dockerfile(s)

[ImreSamu]

fixing https://registry.hub.docker.com/r/postgis/postgis/ (README) - Dockerfile links

• with adding the full path
  • now: [Dockerfile](./14-3.2/Dockerfile)
    • and redirecting to the not exists https://registry.hub.docker.com/r/postgis/postgis/14-3.2/Dockerfile
  • proposed: [Dockerfile](https://github.com/postgis/docker-postgis/blob/master/14-3.2/Dockerfile)

imho: hardcoding the "master" branch is not a perfect solution, but it's the best I can come up with. ( and no problem if not accepted )

And the usual make update - updating *-master Dockerfile(s)

if the CI/CD ok: ready to review/merge/reject