Deprecate CalcCenterOfMassPosition() in favor of CalcCenterOfMassPositionInWorld().

[mitiguy]

While working on #14268, we decided that for a consistent API for center of mass methods, it would be beneficial to deprecate CalcCenterOfMassPosition() in favor of CalcCenterOfMassPositionInWorld().

As decided in issue #14268, the public API for center of mass methods names are (or will be):
Vector3 CalcCenterOfMassPositionInWorld(const systems::Context& context) const;
Vector3 CalcCenterOfMassVelocityInWorld(const systems::Context& context) const;
Vector3 CalcCenterOfMassAccelerationInWorld(const systems::Context& context) const;

Vector3 CalcCenterOfMassPositionInWorld(const systems::Context& context,
const std::vector& model_instances) const;
Vector3 CalcCenterOfMassVelocityInWorld(const systems::Context& context,
const std::vector& model_instances) const;
Vector3 CalcCenterOfMassAccelerationInWorld(const systems::Context& context,
const std::vector& model_instances) const;

[mitiguy]

Created PR #14538

[mitiguy]

Note: I created the file CppPythonDeprecateAP as an example of how to deprecate a C++ method which has a corresponding Python API. That example draws on this PR #14538.  It relates to issue #10605. Although the goal of this PR #14538 was a simple name change, the deprecation process required many (non-obvious) steps.