Replace boost::totally_ordered with explicit operator overloads for SdfListProxy

[nvmkuruc]

Description of Change(s)

• Explicitly implement operator{!=,<=,>=,>} for SdfListProxy::_ItemProxy, following the pattern of using value_type as the operand established by operator== and operator<.
• Explicitly implement operator{!=,<=,>=,>} for SdfListProxy when value_vector_type is the operand

Fixes Issue(s)

• boost::totally_ordered can be replaced with explicit operator overloads #2250

• I have verified that all unit tests pass with the proposed changes

• I have submitted a signed Contributor License Agreement

[tallytalwar]

Filed as internal issue #USD-8022