workflow updates

[skeydan]

Hi,

I've made a few changes to the workflow files:

• updated Ubuntu version
• using @v2 everywhere
• using setup-r-dependencies (https://github.com/r-lib/actions/tree/v2/setup-r-dependencies#readme) to replace manual installation of R packages (I do hope though that in this process, I didn't remove anything that is still needed :-))

Please review and let me know of problems/changes to make :-)

BTW, could someone who has the necessary privilege do the master -> main renaming? That would be great.

[dfalbel]

Thanks @skeydan ! I merged the PR because for some reason the workflow has not been triggered by the PR. Hopefully it will be triggered in future pull requests now that it's enabled again.