Document dependency on R 4.1

[Bisaloo]

#249 introduced the use of the base pipe |>, which is only available since R 4.1.

This means that serofoi now depends on R >= 4.1.0.

Note that this means that serofoi is not completely aligned with Epiverse & the tidyverse's standards in terms of R version support. This is not a big deal but it might mean that some users will be forced to upgrade their R version to use serofoi.

I think it can stay this way for simplicity and because the new version of R should be release in < 6 months and make serofoi compliant with our policy again.

However, the dependency definitely needs to be documented as serofoi currently reports it is compatible to versions as old as R 3.5.0:

serofoi/DESCRIPTION

Lines 21 to 22 in 4be70e3

	Depends:
	R (>= 3.5.0)

Adding the oldest supported version to the CI matrix can also be a good way to make sure the declared oldest supported version is indeed compatible with the package:

serofoi/.github/workflows/R-CMD-check.yaml

Lines 55 to 61 in a608a0d

	matrix:
	config:
	- {os: macOS-latest, r: 'release'}
	- {os: windows-latest, r: 'release'}
	- {os: ubuntu-latest, r: 'devel', http-user-agent: 'release'}
	- {os: ubuntu-latest, r: 'release'}
	- {os: ubuntu-latest, r: 'oldrel-1'}

Please let me know if you'd like a PR for this. And if so, please let me know which branch should be targeted.

[ntorresd]

Hi @Bisaloo, thanks for pointing this out. You can target main now.