3.2.0: back out changes to Path Item Object and bring its `$ref` field back

[ralfhandl]

Fixes #3734 by

• adding $ref field back to Path Item Object
• removing Reference Object as an alternative to Path Item Object in four places

Builds on and includes

• #4066

[handrews]

@ralfhandl I'll look at this once the "built on" stack is resolved :-)

[ralfhandl]

I'll look at this once the "built on" stack is resolved

In case you are curious, these are the changes: https://github.com/ralfhandl/OpenAPI-Specification/compare/3.2.0-sync-with-3.1.1...3.2.0-back-out-changes-to-path-item-object

• bring back $ref field in Path Item Object
• remove Reference Object as an alternative to Path Item Object in four places

[handrews]

@ralfhandl if we follow the proposal in https://github.com/OAI/OpenAPI-Specification/issues/3677#issuecomment-2427757769, we will not need this PR as we will just abandon v3.2.0-dev in favor of a reconstructed v3.2-dev, where the reconstruction just omits the commits that made this change that we want to back out.

[ralfhandl]

@handrews Thanks for taking over!