Replace boost::totally_ordered with explicit operator overloads for TfEnum

[nvmkuruc]

Description of Change(s)

• Add explicit implementations of operator<=, operator>, operator>=, and operator!= to TfEnum
• Update doxygen comments so operator!='s references operator== and operator<=, operator>, and operator>= reference operator<
• Add operator test coverage for TfEnum

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-7985