Change what prefix means in the schema

[lmolkova]

Depends on open-telemetry/semantic-conventions#1292 and open-telemetry/semantic-conventions#1293

Once we migrate semaconv tooling to weaver, we can reconsider usage prefix:

• prefix could be a property that's used to validate that all attributes defined in scope of group have this prefix
  • it could be required to embed things and might be optional in other cases
• prefix is removed and only used when embedding things.

[lmolkova]

blocked on

• embedding Embed functionality v0.2 semantic-conventions#1264
• moving to weaver-based codegen and other tooling

[jsuereth]

Let's remove prefix form weaver in --future mode.