Deprecate MeasurementProcess.return_type and ObservableReturnTypes

[JerryChen97]

Context:
The MeasurementProcess.return_type is only used by the legacy device in a chain of if mp.return_type.name is ... checks that can easily be replaced with isinstance checks.

In PL0.40, return_type is a property of MeasurementProcess which returns None only. Meanwhile, ObservableReturnTypes is an Enum that defines and wraps the name strings, redundantly.

Description of the Change:

• raise deprecation warnings in return_type.
• replace legacy device usages of return_type with direct isinstance check
• unwrap ObservableReturnTypes with direct definitions.

Benefits:
Less redundancies.

Possible Drawbacks:
any downstreaming usage of the deprecations:

• lightning
• catalyst
• qml
• plugins

Related GitHub Issues:
[sc-71892]

[github-actions]

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