upgrade github pages action to ghactions 2.0

[maxheld83]

trafficstars

Helpful for publishing pkgdown docs, as well as all sorts of other projects.

There doesn't yet appear to be "native" support for this from GitHub, though it might make sense to ask them whether this is planned. If it is planned, might as well save the work.

This action lives here: https://github.com/maxheld83/ghpages Placeholder for https://github.com/maxheld83/ghpages/issues/21

[maxheld83]

just asked support whether this is on their roadmap, in which case it might not make sense to tackle this.