Add backwards compatibility to qml.generator()

[josh146]

Context:

• In previous versions of PennyLane, Operator.generator was a property, that returned a list of the form [type, float] or [array, float].

• In the current master, this has been changed. Operator.generator is now a method, and it returns an Operator instance.

• In Add qml.generator(op) function #2256, a high-level qml.generator() function was added, intended to be the user-facing interface to Operator.generator().

Description of the Change: A small modification to qml.generator() has been made, to ensure it continues to work with old-style operators that define the Operator.generator property. In addition, a deprecation warning is raised.

Benefits: qml.generator() should work with old-style operators.

Possible Drawbacks: This backwards compatibility is constrained, in that it requires developers start using qml.generator, a new function!

Related GitHub Issues: n/a

[github-actions]

Hello. You may have forgotten to update the changelog!
Please edit doc/releases/changelog-dev.md with:

• A one-to-two sentence description of the change. You may include a small working example for new features.
• A link back to this PR.
• Your name (or GitHub username) in the contributors section.

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (v0.22.0-rc0@78447d0). Click here to learn what that means.
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❗ Current head f113d1f differs from pull request most recent head cee4c03. Consider uploading reports for the commit cee4c03 to get more accurate results

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