Update SingularityCE/Apptainer version used in this reference version of the course

[jcohen02]

As detailed in the discussions in #9 and #32, the version of Singularity used in this reference version of the course material is now significantly out of date. Since the course was written, Singularity has been forked and gone through some changes as the open source versions of the tool have become SingularityCE and Apptainer.

With there being some uncertainty around the changes and which future version of the tool to base the course around, we opted to leave this reference version of the material in it's current state. However, things may now have reached a more stable state where we can make an update to the reference material.

When re run courses based on this material, the approach taken is to fork the repo and then make updates specific to the instance of the course being run. This is because timing and material covered changes between courses but also because courses tend to be run using different HPC platforms which require different instructions, notes, etc, so we update the specific course instance to reflect the target platform for that session. There have been updates to more recent runs of the course to use much more recent releases of SingularityCE or Apptainer (e.g. https://epcced.github.io/2023-09-06-rse-singularity/setup.html) and now may be an ideal time to address issues around this and update the reference material to focus on a newer release.