doc: normalize C++ code block info strings

Prior to this commit, C++ fenced code blocks in Markdown files had
inconsistent info strings. This has been corrected to standarize on
the one with the highest frequency in the doc/api/ dir.

Stats:
>  'cpp' => 19,
>  'C++' => 6,
>  'c++' => 3,

PR-URL: https://github.com/nodejs/node/pull/33483


Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: David Carlier <devnexen@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>