If the attributes are NOT used, make `relay` to rename those attributes

[antonpirker]

(we can also decide to "break" the users and do this if users have those attributes in their dashboards, or saved queries)

The attributes can easily be renamed in relay. We need to set the old name as legacy_alias and the new name as field like is shown here: https://github.com/getsentry/relay/blob/master/relay-event-schema/src/protocol/span.rs#L544-L550

This will rename the field, so the legacy field name will not be available in the saved span.

For span.op and span.name it is more work, because we need to find the place in relay where we can do this, for this we need the ingest team.