Switch to R 4 for CI/CD

[riccardoporreca]

Controlling the package dependencies and R version is crucial to a smooth collaborative development and safe deployments. See also #116 for a recent breakage caused by picking the latest shiny 1.5.0

• R version: Note that shinyapps.io serves the app using the R version used when running rsconnect::deployApp(), so we are not bound to the latest R version and can set it explicitly in the setup-r action as r-version~R 4.0.0 is out, and shinyapps.io will be updated accordingly soon (expected by around May 3rd, see https://community.rstudio.com/t/shinyapps-io-cant-deploy-apps-built-with-r-4-0-0/63143/3)~

• Package dependencies: We can use full-control via renv (as in https://github.com/miraisolutions/eRum2020Workshop) and / or rely on an MRAN / RSPM snapshot => renv setup as part of #151

As part of this, we should also review the CI-CD Actions workflow WF and setup following https://github.com/miraisolutions/eRum2020Workshop (which includes making sure the cache is Rmajor.minor-specific), and adapt it to the agreed package dependencies control.