[libc++][docs] Fix documentation of REQUIRES: std-at-least-* (#166221)

Author: vogelsgesang

libcxx/docs/TestingLibcxx.rst

@@ -451,7 +451,7 @@ Instead use:
 
 .. code-block:: cpp
 
-   // UNSUPPORTED: std-at-least-c++26
+   // REQUIRES: requires-std-at-least-c++26
 
 There is no corresponding ``std-at-most-c++23``. This could be useful when
 tests are only valid for a small set of standard versions. For example, a