Revisiting native pipe (and lambda) use in ggraph

[malcolmbarrett]

Last year in #364, ggraph reverted its use of the native pipe to support older versions of R. Now that we are a year further in the tidyverse-sanctioned four years of support, R 4.1.0 is the lowest supported version of R.

I have decided to switch ggdag to the native pipe and lambda (r-causal/ggdag#192), bumping the required version to R 4.1.0 (although I am still exporting the native pipe because we depend on dplyr which still exports it, so this will help prevent breaking scripts).

As such, ggdag is no longer a bottleneck in ggraph converting to the native pipe. I can't speak to other users, but if you would like to go ahead with the conversion, we will no longer hold you up!