[ci] [R-package] add arm64 macOS R-package CI job (fixes #6481)

[jameslamb]

Fixes #6481

Proposes adding an arm64 macOS CI job testing the R package. CRAN checks every submission on an arm64 Mac (https://cran.r-project.org/web/checks/check_flavors.html), so hopefully this will help catch issues before attempting submissions to CRAN.

To avoid further growing the CI matrix, this proposes dropping the job that tests the R package compiled with clang on Linux with R 3.6. I think that's ok, because there are others covering compiling the R package with gcc on Linux

LightGBM/.github/workflows/r_package.yml

Lines 58 to 63 in fc788a5

	- os: ubuntu-latest
	task: r-package
	compiler: clang
	r_version: 3.6
	build_type: cmake
	container: 'ubuntu:18.04'

And because we're planning to move away from supporting R 3.6: #6442 (comment)

This should also reduce the manual testing effort for releases: #6439 (comment)