Support C++20 and C++23 (in r-devel)

[eddelbuettel]

r-devel now support C++23 as a compilation standard (if and when the compiler does) so we added a plugin. We also added one for C++20 which we had apparently skipped.

Tested both under r-devel (svn r82883) and worked.

Checklist

• Code compiles correctly
• R CMD check still passes all tests
• Prefereably, new tests were added which fail without the change
• Document the changes by file in ChangeLog

[Enchufa2]

LGTM. Just note that there's already a cpp2a plugin

Rcpp/R/Attributes.R

Lines 572 to 574 in bb7cebb

	.plugins[["cpp2a"]] <- function() {
	list(env = list(PKG_CXXFLAGS ="-std=c++2a"))
	}

so we could add the cpp2b too for completeness.

[eddelbuettel]

Yup -- added '2b' for symmetry as well. Helps cover more compilers -- thanks for patiently arguing for that.

[kevinushey]

Belated LGTM!