Replace _compat.cached_property with functools.cached_property

[pavoljuhas]

Description of the issue

cirq uses an internal version of cached_property defined here to support earlier versions of Python.

Cirq/cirq-core/cirq/_compat.py

Lines 64 to 67 in d33b1a7

	try:
	from functools import cached_property # pylint: disable=unused-import
	except ImportError:
	from backports.cached_property import cached_property # type: ignore[no-redef]

As of #6167 we require Python 3.9 which is guaranteed to have functools.cached_property.

We can therefore replace all instances of _compat.cached_property with functools.cached_property and remove the cached_property definition from the _compat.py.

Cirq version

1.4.0 at d33b1a7

[Josha91]

I opened a PR for this: #6398.

I closed the first one because it was committed with the wrong e-mail address (and making a dummy commit with the right e-mail still failed the CLA, so I figured opening a new PR was simpler).