Update default documentation for DS_PIP_VERSION for dependency scanning

What does this MR do?

This MR updates the docs for DS_PIP_VERSION (added in !21218 (merged)) to remove the default value and indicates that this version will force the install of a new pip version.

Relevant issue: #12811 (closed)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• [-] Code review guidelines
• [-] Merge request performance guidelines
• [-] Style guides
• [-] Database guides
• [-] Separation of EE specific content