Fix : Replace deprecated documentDescribes with DESCRIBES relationships in SPDX 2.3+

[guptapratykshh]

Description

The documentDescribes field is deprecated in SPDX 2.3+ and should not be used. Per the SPDX 2.3.1 schema, relationships should be used instead to describe relationships between packages in the SBOM.
Reference: https://github.com/spdx/spdx-spec/blob/support/2.3.1/schemas/spdx-schema.json

Solution

• For SPDX 2.3+: Removed documentDescribes field, added DESCRIBES relationships from SPDXRef-DOCUMENT to described elements
• For SPDX 2.2: Kept documentDescribes for backward compatibility
• Reading documents: Updated Document.from_data() to handle both old (documentDescribes) and new (DESCRIBES relationships) formats

Fixes #1958

[guptapratykshh]

please review this @tdruez