Deprecate qiskit/transpiler/synthesis and move to qiskit/synthesis (backport #11426)

[mergify]

This is an automatic backport of pull request #11426 done by Mergify.
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Changes to be committed:
	modified:   docs/apidoc/synthesis_aqc.rst
	modified:   qiskit/circuit/library/generalized_gates/linear_function.py
	new file:   qiskit/synthesis/unitary/__init__.py
	new file:   qiskit/synthesis/unitary/aqc/__init__.py
	renamed:    qiskit/transpiler/synthesis/aqc/approximate.py -> qiskit/synthesis/unitary/aqc/approximate.py
	renamed:    qiskit/transpiler/synthesis/aqc/aqc.py -> qiskit/synthesis/unitary/aqc/aqc.py
	renamed:    qiskit/transpiler/synthesis/aqc/cnot_structures.py -> qiskit/synthesis/unitary/aqc/cnot_structures.py
	renamed:    qiskit/transpiler/synthesis/aqc/cnot_unit_circuit.py -> qiskit/synthesis/unitary/aqc/cnot_unit_circuit.py
	renamed:    qiskit/transpiler/synthesis/aqc/cnot_unit_objective.py -> qiskit/synthesis/unitary/aqc/cnot_unit_objective.py
	renamed:    qiskit/transpiler/synthesis/aqc/elementary_operations.py -> qiskit/synthesis/unitary/aqc/elementary_operations.py
	renamed:    qiskit/transpiler/synthesis/aqc/fast_gradient/__init__.py -> qiskit/synthesis/unitary/aqc/fast_gradient/__init__.py
	renamed:    qiskit/transpiler/synthesis/aqc/fast_gradient/fast_grad_utils.py -> qiskit/synthesis/unitary/aqc/fast_gradient/fast_grad_utils.py
	renamed:    qiskit/transpiler/synthesis/aqc/fast_gradient/fast_gradient.py -> qiskit/synthesis/unitary/aqc/fast_gradient/fast_gradient.py
	renamed:    qiskit/transpiler/synthesis/aqc/fast_gradient/layer.py -> qiskit/synthesis/unitary/aqc/fast_gradient/layer.py
	renamed:    qiskit/transpiler/synthesis/aqc/fast_gradient/pmatrix.py -> qiskit/synthesis/unitary/aqc/fast_gradient/pmatrix.py
	modified:   qiskit/transpiler/passes/__init__.py
	modified:   qiskit/transpiler/passes/synthesis/__init__.py
	modified:   qiskit/transpiler/synthesis/aqc/__init__.py
	deleted:    qiskit/transpiler/synthesis/aqc/aqc_plugin.py
	modified:   qiskit/transpiler/synthesis/graysynth.py
	new file:   releasenotes/notes/deprecate-transpiler-synthesis-cf4e6e6dcdb42eca.yaml
	renamed:    test/python/transpiler/aqc/__init__.py -> test/python/synthesis/aqc/__init__.py
	renamed:    test/python/transpiler/aqc/fast_gradient/__init__.py -> test/python/synthesis/aqc/fast_gradient/__init__.py
	renamed:    test/python/transpiler/aqc/fast_gradient/test_cmp_gradients.py -> test/python/synthesis/aqc/fast_gradient/test_cmp_gradients.py
	renamed:    test/python/transpiler/aqc/fast_gradient/test_layer1q.py -> test/python/synthesis/aqc/fast_gradient/test_layer1q.py
	renamed:    test/python/transpiler/aqc/fast_gradient/test_layer2q.py -> test/python/synthesis/aqc/fast_gradient/test_layer2q.py
	renamed:    test/python/transpiler/aqc/fast_gradient/test_utils.py -> test/python/synthesis/aqc/fast_gradient/test_utils.py
	renamed:    test/python/transpiler/aqc/fast_gradient/utils_for_testing.py -> test/python/synthesis/aqc/fast_gradient/utils_for_testing.py
	renamed:    test/python/transpiler/aqc/sample_data.py -> test/python/synthesis/aqc/sample_data.py
	renamed:    test/python/transpiler/aqc/test_aqc.py -> test/python/synthesis/aqc/test_aqc.py
	renamed:    test/python/transpiler/aqc/test_aqc_plugin.py -> test/python/synthesis/aqc/test_aqc_plugin.py
	renamed:    test/python/transpiler/aqc/test_cnot_networks.py -> test/python/synthesis/aqc/test_cnot_networks.py
	renamed:    test/python/transpiler/aqc/test_gradient.py -> test/python/synthesis/aqc/test_gradient.py
	modified:   test/python/synthesis/test_cnot_phase_synthesis.py

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   pyproject.toml
	both modified:   qiskit/synthesis/__init__.py
	both modified:   qiskit/transpiler/passes/synthesis/aqc_plugin.py

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/github/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

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