XML package requires 4.0

[benkates]

app_03 requires XML and since the guidance is to use R version 3.6 this requires changing the version of R used. Becomes a bit of a roadblock when working through the training docs.

[andrie]

Thanks for opening this issue. The new version of the course instructs to use R-4.0, so this should no longer be an issue.

However, I'm keeping this issue open, since I'd prefer to use xml over XML.