[C++] Switch `flatc` to `--cpp-std c++11` C++ code generator (#6306)

[C++] Switch `flatc` to `--cpp-std c++11` C++ code generator (#6306)

Prior to this commit the default C++ code generator was `c++0x`.
A code generated with `c++0x` code-gen might have a vulnerability (undefined behavior) connected evolution of enums in a schema. This UB could break the backward compatibility if previously generated code casts an unknown enumerator to enum type that knows nothing about future enumerators added to the schema.

The main differences between `c++0x` and `c++11`:

- generated enums use explicitly declared underlying type;
- generated object-API tables don't declare default ctor() explicitly, instead of it default data member initializers are generated.

Please use `flatc --cpp-std c++0x` option for backward compatibility with old compilers.