Singularity has been renamed to Apptainer

See https://apptainer.org/news/community-announcement-20211130

The action should have support for Apptainer once a [release](https://github.com/apptainer/apptainer/releases) for it has been made.

We should make a fork of this action to new action called `setup-apptainer` to make clear the action is for apptainer and not for the singularity products offered by the syslabs company.

The https://github.com/hpcng/singularity repo has been moved to https://github.com/apptainer/singularity, it has the old singularity versions and should be used to download+build from. The repo URL should be changed in this repo and in https://github.com/eWaterCycle/singularity-versions