Dependency `purrr` requires R >= 4.1

[RasmusSkytte]

Describe the bug
We use purrr quite extensively but also claim to support R >= 3.6.

Since purrr is updated, I suggest we bump our dependency R >= 4.1 and revert some of the changes of #165.

To Reproduce
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Expected behaviour
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Screenshots
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Provide output from sessionInfo()
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• I have checked that the bug is not already described in open issues

Additional context
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