Release publisher extension: decide what to do in OCDS 1.2

[yolile]

As per open-contracting/standard#325, the publisher field will be part of the standard itself, so as mentioned in open-contracting/standard#1376 (comment), we need to decide if we should de-register the release publisher extension or add a warning that it is incompatible with 1.2

[jpmckinney]

Noting that the issue is an inconsistency. The extension uses scheme, id, name, uri, but OCDS 1.2 uses the Identifier definition (scheme, id, legalName, uri).

[duncandewhurst]

open-contracting/ocds-extensions#202 proposes adding a note to this extension's documentation, saying to only use it in OCDS 1.1. We can perhaps add a particularly strongly worded note given the inconsistency.