Require R4.0

[nikosbosse]

Description

As mentioned by @Bisaloo in #813, tests on R 3.6 are currently failing, because we have a dependency that requires R 4.0. Since tidyverse also started requiring R 4.0, there is maybe not that much reason to keep supporting R 3.6 anymore.

Winbuilder is also only checking

R-oldrelease: R-oldrelease, currently R-4.3.3
R-release: R-release, currently R-4.4.0
R-devel: R-devel, to be R-4.5.0

Checklist

• [ ] My PR is based on a package issue and I have explicitly linked it.
• [x] I have included the target issue or issues in the PR title as follows: issue-number: PR title
• [x] I have tested my changes locally.
• [x] I have added or updated unit tests where necessary.
• [x] I have updated the documentation if required.
• [x] I have built the package locally and run rebuilt docs using roxygen2.
• [x] My code follows the established coding standards and I have run lintr::lint_package() to check for style issues introduced by my changes.
• [x] I have added a news item linked to this PR.
• [ ] I have reviewed CI checks for this PR and addressed them as far as I am able.

[nikosbosse]

Can this be merged?