deprecated metadata on operation broken in C++

[bernardnormier]

There are at least two issues:

• the generated code does not compile when an operation is marked deprecated

src/IceGridLib/generated/Admin.cpp:5498:11: error: 'keepAlive' is deprecated: is deprecated [-Werror,-Wdeprecated-declarations]
    this->keepAlive(request.current());

From:

["deprecated"] idempotent void keepAlive();

• the C++ deprecated attribute message with no message should not be "is deprecated":

 [[deprecated("is deprecated")]] virtual void keepAlive(const ::Ice::Current& current) = 0;

[externl]

Once this is fixed we also need to do: https://github.com/zeroc-ice/ice/pull/2087#pullrequestreview-2031264169

[InsertCreativityHere]

This was fixed in #2288.

[InsertCreativityHere]

But I'm leaving this open, because it's still broken in other languages. Once it's 'un-broken' it sounds like we want to deprecate: Glacier2::Router::refreshSession and IceGrid::Admin::keepAlive

I suspect we'll also want to deprecate IceGrid::Session::keepAlive.

Anything else?