SDMN — XML Namespace URI needs updating

The XML namespace URI of the original SDMN submission uses a dated version stamp, which if changed with each revision could lead to model interchange issues, i.e. different tools supporting different versions with incompatible XML namespaces (see also SCE-117).

In addition, the following sentence about imports was accidentally copied from DMN is invalid for SDMN:

SDMN files MAY import non-SDMN files (such as XSDs and PMMLs) if the contained elements use external
definitions.

Use fixed Namespace URI and xsi:schemaLocation

Use the fixed Namespace URI https://www.omg.org/spec/SDMN/ (without a dated version stamp as permitted by section 3.1.2 and annex A of the OMG Policy for Versioning of Specification URIs, File URIs, and XML Namespaces (smsc/2018-08-01)) and adopt the mechanism introduced by SCE-117 to allow tool interoperability between backwards-compatible SDMN versions.

In addition, an invalid sentence about imports that was accidentally copied from DMN is removed.