[ CI ] Currently broken presumably due to old version of `upload-pages-artifact`

[CodingCellist]

Trying to breathe some life back into not having the website have a bus factor of 1 / moving it under the idris-lang org from Edwin's personal repo. However, it seems CI is currently blocked on an old version of the deployment to GH Pages (understandable, since it seems it was ~3 years ago that anyone last ran the CI). The current version at the time of writing seems to be actions/deploy-pages@v4, but I don't know if it is a drop-in solution or if variables/configs need updating, so I'm creating this issue to not forget.