Change `enduser.*` attributes to `user.*` in 3.0

[trask]

These were changed in open-telemetry/semantic-conventions#731

enduser.id was brought back, but in the context of client instrumentation, I think we should probably switch our usage to user.id

enduser.scope was deprecated with no replacement, will need to consider what to do with that

[jaydeluca]

should we wait until we are closer to 3.0 or do you think it would be ok to make this change sooner?