[3.9] Fix qft function

[lucasvanmol]

Changes made

Fix the qft function to work for n != 4

Justification

If someone wants to vary the number of counting qubits, the function fails. Note that simply varying t a few cells below still does not work due to the hardcoded 4 in this cell:

qft_dagger = qft(4).to_gate().inverse()
qft_dagger.label = "QFT†"

although generalising to use the t variable instead would require some re-ordering.

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[frankharkins]

Good catch, thanks!