[libcxx] Update include/__libcpp_version to match include/__config

https://reviews.llvm.org/rG5369517d20dd362a178a1b2d6c398d8898ee4620
bumped the version number in __config to 13000, causing a test failure
in libcxx/test/libcxx/libcpp_version.pass.cpp because now the two
don't match.

This is the only part of the post-release TODO in
libcxx/docs/Contributing.rst that wasn't done by that commit.