Rename development branch to `main`

[rhusar]

GitHub and others have moved on to name the development branch to main.

Consider doing the same for this repo.

https://sfconservancy.org/news/2020/jun/23/gitbranchname/a

[BalusC]

main is the stable branch btw, develop is the development branch (in Gitflow, that is).

Well, I've already in mind to rename/rework the branches, also to align more with jakartaee/faces repo where master is actually the current release.