Check pandoc version requirements and templates

[cderv]

It seems all templates does not support old Pandoc version (before 2.7.3): https://github.com/rstudio/rticles/runs/2322462437?check_suite_focus=true

Fails with 2.5 and 2.0.0.1 for example.

There was no issue reported by users so maybe we should not bother but it is good to now that rticles has a stronger requirement than rmarkdown