Use of deprecated field to state relationships in SBOM

**Describe the bug**
documentDescribes is a deprecated field and should not be used to describe relationships in SBOM. Instead relationships array should be used.

**System configuration**
- Which version of ScanCode.io are you running?
- Are you running the app using Docker?
- On which OS?
- What inputs are you using?
- Which pipeline are you running?

**To Reproduce**
Steps to reproduce the behavior:
1. Go to '...'
2. Click on '....'
3. Scroll down to '....'
4. See error

**Expected behavior**
As per SPDX schema, documentDescribes is a deprecated field and relationships array should be used to describe relationship b/w different packages of the SBOM.

https://github.com/spdx/spdx-spec/blob/support/2.3.1/schemas/spdx-schema.json

**Screenshots**
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