Need to refactor or delete all code from flagsmith, flagsmith-common and flagsmith-workflows that's associated with the old segment serializer and model APIs.

Task list

• Delete unused metadata and segment serializer code from flagsmith-common.
  • ✅ Completed as of MetadataSerializers of core and flagsmith-common clashing #5907.
• Delete obsolete logic from flagsmith-workflows.
  • ✅ Completed as of https://github.com/Flagsmith/flagsmith-workflows/pull/72.
• Import improvements from this closed PR:
  • Segment manager/model architecture
  • Segment tests
  • ✅ Completed in refactor(Segments): Clean up after Segment Change Request work #6265
• Make Segment.version.default = 1 at database level instead of using a hook.
  • ✅ Completed in refactor(Segments): Clean up after Segment Change Request work #6265
• Make Segment.version_of = null the indication for a live segment — right now live segments are those which version_of point to themselves, which adds unnecessary overhead and complexity.
  • ✅ Completed in refactor(Segments): Clean up after Segment Change Request work #6265
• Inherit MetadataSerializerMixin from Serializer and maybe prevent repeating meta fields?
  • ✅ Completed in refactor(Segments): Clean up after Segment Change Request work #6265