[libc++][doc] Bump required GCC version.

GCC 13 is the latest GCC release and tested in the libc++ CI for several
month. According to our policy we only support the latest version,
update the documentation to the latest version.

Author: mordante

libcxx/docs/index.rst

@@ -119,7 +119,7 @@ Compiler     Versions        Restrictions               Support policy
 Clang        16, 17, 18-git                             latest two stable releases per `LLVM's release page <https://releases.llvm.org>`_ and the development version
 AppleClang   15                                         latest stable release per `Xcode's release page <https://developer.apple.com/documentation/xcode-release-notes>`_
 Open XL      17.1 (AIX)                                 latest stable release per `Open XL's documentation page <https://www.ibm.com/docs/en/openxl-c-and-cpp-aix>`_
-GCC          12              In C++11 or later only     latest stable release per `GCC's release page <https://gcc.gnu.org/releases.html>`_
+GCC          13              In C++11 or later only     latest stable release per `GCC's release page <https://gcc.gnu.org/releases.html>`_
 ============ =============== ========================== =====================
 
 Libc++ also supports common platforms and architectures: