need to depend on R >= 4.1.0

When checking reverse deps of nimble, I see this, as something to fix in the next release.

```
* checking DESCRIPTION meta-information ... NOTE
  Missing dependency on R >= 4.1.0 because package code uses the pipe
  |> or function shorthand \(...) syntax added in R 4.1.0.
  File(s) using such syntax:
    ‘compareMCMCs.R’ ‘metrics-addMetrics.R’
```