[.NET Migration Guide Revamp] [Phase 1] Move content 'Retarget to .NET Framework 4.7.2' content from 'Port libraries' page.

[StephenBonikowsky]

For details on revamp see: #23561
New TOC: #23215

• Retarget to .NET Framework 4.7.2 (MOVE) This section needs to be moved to a new location in the new TOC and updated as needed.

• name: Migration guide
  items:

  • name: Overview
    href: ../core/porting/index.md
  • name: General Information
    items:
  • name: Pre-Migration
    items:
    • name: Needed changes before porting code
      href: ../core/porting/premigration-needed-changes.md

[StephenBonikowsky]

Completed with #24630