Update UnitaryGate import now that qiskit.extensions is deprecated

[garrison]

qiskit.extensions is deprecated starting with Qiskit 0.45, and the UnitaryGate class has moved to qiskit.circuit.library. However, this class did not exist at this location prior to Qiskit 0.45, so this means having to bump the minimum Qiskit version as well.

[coveralls]

Pull Request Test Coverage Report for Build 7202329089

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 92.498%

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Totals
Change from base Build 7189163548:	0.0%
Covered Lines:	2984
Relevant Lines:	3226

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💛 - Coveralls

[garrison]

If we merge this before the next release, it will allow us to knock out a few DeprecationWarnings at the cost of requiring a more recent qiskit (i.e., one released in November rather than August). I don't think this is a huge down side, but there's also no urgency to merge this before the next release. (We'll definitely have to merge it before CKT can work with Qiskit 1.0, however.)