Deprecate cirq.ion module

[tanujkhattar]

Fixes #2793
Fixes #5128

[MichaelBroughton]

LGTM, do we want to deprecate cirq.ion like we did with cirq.google or does getting all the major things inside of it seem like enough ?

[tanujkhattar]

do we want to deprecate cirq.ion like we did with cirq.google or does getting all the major things inside of it seem like enough

Since we have deprecated everything inside the module, we don't need to deprecate the cirq.ion submodule itself. The latter is needed when we are moving utilities but want the users to update the path (for example, use cirq_google.Sycamore instead of cirq.google.Syamore). In this case, no object inside the cirq.ion is preserved -- everything is replaced by newer alternatives and therefore just deprecating all the objects is enough.