[R-package] [ci] drop support for R 3.x (fixes #7023)

[jameslamb]

Fixes #7023

Proposes the following:

• moving floor for the R package to R (>= 4.0)
• using C++17 unconditionally in the R package
  • this change only affects build from source on Windows
  • all other builds (and all CRAN builds) have been using C++17 since January 2023 ([R-package] use C++17 in the CRAN package #5690)

Notes for Reviewers

In an earlier attempt (#7025), I also tried to update to testing on both R 4.0 and R 4.5.
Ran into some issues there on Windows... I think that Rtools have changed in significant ways between R minor versions.

So proposing this PR to get the easier-to-implement "just drop R 3.x" changes merged, to help unblock #7016

[jameslamb]

@jmoralez since you're familiar with the R CI here, could you help review this?

[jameslamb]

thank you!!