[libc++] Implement P1423R3 (char8_t backward compatibility remediation)

Reviewed By: Mordante, #libc

Spies: h-vetinari, libcxx-commits, arichardson

Differential Revision: https://reviews.llvm.org/D129195

NOKEYCHECK=True
GitOrigin-RevId: 309aed306817e5b1a302d8f757231c734cf71370
diff --git a/docs/FeatureTestMacroTable.rst b/docs/FeatureTestMacroTable.rst
index f615b40..7ab2d6c 100644
--- a/docs/FeatureTestMacroTable.rst
+++ b/docs/FeatureTestMacroTable.rst
@@ -196,7 +196,7 @@
     ------------------------------------------------- -----------------
     ``__cpp_lib_bounded_array_traits``                ``201902L``
     ------------------------------------------------- -----------------
-    ``__cpp_lib_char8_t``                             ``201811L``
+    ``__cpp_lib_char8_t``                             ``201907L``
     ------------------------------------------------- -----------------
     ``__cpp_lib_concepts``                            ``202002L``
     ------------------------------------------------- -----------------