remove recommendation to specify SystemRequirements: C++11

[romainfrancois]

closes #314

[jennybc]

LGTM (although I am not expert on all the various places this could have been mentioned).

[romainfrancois]

cc @ateucher this was based on your article: https://www.tidyverse.org/blog/2023/03/cran-checks-compiled-code/#note-regarding-systemrequirements-c11

[ateucher]

Thanks @romainfrancois - the advice in that article was summarized from several internal Slack conversations as well as some discussions on R-pkg-devel, and I think has probably had enough road testing by now by to be considered pretty robust. Is there anywhere in the docs to say that there may be instances where authors do need to specify the C++ standard in SystemRequirements, if they are using features of a particular C++ standard?

I also wonder if it's worth clarifying somewhere that authors aren't bound to C++11 when using cpp11 (i.e., to circumvent questions like #7 and #100).

[romainfrancois]

Thanks @ateucher. I will leave that to @DavisVaughan for future versions, in the interest of releasing this week.