Declare C++11 as a requirement in the library metadata

The metadata advertised C++03, but the archives have needed C++11 for
some time: including boost/archive/text_oarchive.hpp pulls in
boost/scoped_ptr.hpp, and Boost.SmartPtr uses `noexcept`.  Nothing
tested C++03 either, as every CI job starts at 11.

Author: gennaroprota

meta/libraries.json

@@ -11,5 +11,5 @@
     "maintainers": [
         "Robert Ramey <ramey -at- rrsd.com>"
     ],
-    "cxxstd": "03"
+    "cxxstd": "11"
 }