Consider deprecating use of dag keyword in DAGNode constructors

[jakelishman]

          Shall we take a note to issue a deprecation warning on the use of the `dag` keyword argument in Qiskit 1.4, and document + release note that the argument is unused as of 1.3?

Originally posted by @jakelishman in #12550 (comment)

The dag argument to DAGNode subclasses is irrelevant since the move of the DAGCircuit to Rust space, so the argument no longer has a meaning. It's a candidate for deprecation in 1.4 and removal in 2.0.