Rename qubitstatevector to stateprep

[multiphaseCFD]

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Context:

Description of the Change:

• Introduce StatePrep in lieu of QubitStateVector in LightningGPU following recent changes in PennyLane PR#4450.
• Deprecate Python 3.8.
• Update Lightning imports.

Benefits:

Possible Drawbacks:

Related GitHub Issues:

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Please edit .github/CHANGELOG.md with:

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[mlxd]

Great work here folks

[vincentmr]

Approved ;)

[AmintorDusko]

Looks good to me!