DDSSEC13 — Modify XSD to make the elements i version 1.2 optional

DDS Security 1.2 added two elements to the Governance XSD: enable_key_revision and rtps_psk_protection_kind.
These elements were added without specifying a value for minOccurs which according the XSD rules it would default to 1. The result of this is that existing governance documents would not be valid according to the XSD. This was not the intent

Instead the elements should have been added specifying minOccurs="0".

This change impacts the machine-readable file omg_shared_ca_governance.xsd