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 standardize on
the lowercase variant, which matches the others, is the typical file
extension for C files, and is the CSS class used by highlight.js
despite representing a minority of appearances in the doc/api/ dir.

Stats:
>  'C' => 181,
>  'c' => 3,

Refs: highlightjs/highlight.js#2577

PR-URL: #33507
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>