[Chapel Merge] Remove mentions of `c_string` in extern technote

Branch: refs/heads/main
Revision: 6bf5ca74e55b540fc1b4e758b2450c83ea9643b5
Author: arifthpe
Link: Remove mentions of `c_string` in extern technote by arifthpe · Pull Request #27230 · chapel-lang/chapel · GitHub
Log Message:
Remove mentions of c_string in extern technote (#27230)

Remove mentions of c_string from the C Interop technote
(extern.rst), as c_string no longer exists.

c_string was removed in
Remove deprecated code from ChapelString, ChapelBase by DanilaFe · Pull Request #26523 · chapel-lang/chapel · GitHub.

[reviewed by @DanilaFe, thanks!]

Compare: Comparing a45af4e5eb4dd23fc23839d3598fa85ac985a919...6edcddbe90c8b3a2db044f58b4163735866ff412 · chapel-lang/chapel · GitHub

Diff:
M doc/rst/technotes/extern.rst
https://github.com/chapel-lang/chapel/pull/27230.diff