Add full deprecation warning to util.py

[jakelishman]

Summary

This causes imports of qiskit.util to raise a DeprecationWarning (it should be qiskit.utils). The module itself has been deprecated since #5420 (released in Terra 0.17), but due to the overrides of the deprecation warning handling (see #7204), we couldn't raise a deprecation warning here without Aer and friends' uses leaking out immediately to users.

I added a reno for this to highlight it; there already was a reno in 0.17, but this might be the first time that downstream packages actually notice the deprecation.

Details and comments

This will cause a few more deprecation warnings to be seen at the top of the test suite (at least until Qiskit/qiskit-aer#1382 is released in a new version), but they won't fail the run because they're triggered during collection, before our test-runner code turns warnings into errors (edit: forgot about Sphinx!).

We shouldn't merge this unless #7204 (at least the _filter_deprecation_warnings bit of it) is in Terra to avoid leaking the extra warnings out to users. Also on hold til Qiskit/qiskit-aer#1382 merges.

[jakelishman]

No longer on hold, because the blockers are merged now.

[coveralls]

Pull Request Test Coverage Report for Build 1477836345

• 2 of 2 (100.0%) changed or added relevant lines in 1 file are covered.
• 3 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-0.004%) to 82.542%

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Files with Coverage Reduction	New Missed Lines	%
qiskit/pulse/library/waveform.py	3	90.38%

Totals
Change from base Build 1476902269:	-0.004%
Covered Lines:	49851
Relevant Lines:	60395

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💛 - Coveralls