Deprecate device.decompose_operation in cirq-core.

[MichaelBroughton]

Yet another deprecation for #4744 . Vendor deprecations of this fxn to follow.

[tanujkhattar]

Many existing vendor devices in cirq use decompose_operation for compiling circuits. What's the plan to update them? We should do the deprecation in cirq-core after the corresponding devices have been updated / an explicit transformer for compilation has been provided?

For example:

• Cirq/cirq-ionq/cirq_ionq/ionq_devices.py

  Line 83 in 2faba7a

  def decompose_operation(self, operation: cirq.Operation) -> cirq.OP_TREE:

• Cirq/cirq-pasqal/cirq_pasqal/pasqal_device.py

  Line 97 in 35e6d21

  def decompose_operation(self, operation: cirq.ops.Operation) -> 'cirq.OP_TREE':

• Cirq/cirq-core/cirq/ion/ion_device.py

  Line 90 in fd4e834

  def decompose_operation(self, operation: ops.Operation) -> ops.OP_TREE:

• Cirq/cirq-google/cirq_google/devices/xmon_device.py

  Line 69 in 12ebe83

  def decompose_operation(self, operation: cirq.Operation) -> cirq.OP_TREE:

• Cirq/cirq-core/cirq/neutral_atoms/neutral_atom_devices.py

  Line 125 in 35e6d21

  def decompose_operation(self, operation: ops.Operation) -> ops.OP_TREE:

[MichaelBroughton]

I'm aware of all the vendors that would need to have this deprecated. For Deprecations where the cirq-core removal would cause breaks or warnings to come up suddenly in the vendor packages we did the vendors first (#4757, #4812, #4820, #4821 all came before #4845 otherwise #4845 would have been huge). In this case since there aren't any breaks and the removals will be pretty straightforward for all the vendors I figured the order didn't matter that much. Would you prefer we still do them first ?

[tanujkhattar]

LGTM