Remove mention of retired IBM devices from user manual and examples

[CalMacCQ]

When demonstrating IBMQBackend in the running on backends section of the manual, we have code snippets for ibmq_quito and ibmq_manilla which are now retired.

See also the examples https://tket.quantinuum.com/examples/spam_example.html https://tket.quantinuum.com/examples/backends_example.html https://tket.quantinuum.com/examples/qiskit_integration.html

Most of the other content in these three examples is overdue for an update as well.

[cqc-melf]

Is this done @CalMacCQ or are we still working on this?

[CalMacCQ]

This is not done yet.