remove check for unavailabe dependencies on ancient R

[ben-schwen]

Current CI ancient job is failing because new version of xts, and evaluate depend on R 4.0.0. This PR patches the CI job by installing fixed versions for certain packages, afterwards.

[MichaelChirico]

This feels a bit fragile to me. I wonder what the practical use case we're testing for is exactly:

1. User on ancient R is also working with ancient {knitr}
2. User on ancient R doesn't need to check vignettes, doesn't care about {knitr}

I've worked towards the latter in the R-CMD-check-occasional GHA, and taken the approach to just skip stuff related to these Suggests deps that won't install:

data.table/.github/workflows/R-CMD-check-occasional.yaml

Lines 144 to 148 in 83fd2c0

	if (!do_vignettes) {
	message("Skipping vignettes since knitr is unavailable.")
	build_args = "--no-build-vignettes"
	check_args = c(check_args, "--no-build-vignettes", "--ignore-vignettes")
	}

(I don't really have a sense one way or the other what's more likely)

[ben-schwen]

1. User on ancient R is also working with ancient {knitr}
2. User on ancient R doesn't need to check vignettes, doesn't care about {knitr}

Number 2 seems the way to go and also simplifies it!

[MichaelChirico]

LGTM! May as well merge #6383 first, right? Since we know CI will fail on 3.3.0 at current master.

[ben-schwen]

LGTM! May as well merge #6383 first, right? Since we know CI will fail on 3.3.0 at current master.

Yes, CI will only deliver new insights after merging #6383