docs(telemetry): drop the stale Jaeger label from the Phase 7 diagram

The Phase 7 section existed twice in this file; the copies disagreed on this
one label. Consolidating on the single copy kept the older wording, but Jaeger
was removed from the project earlier in this chain, so Tempo is the only trace
backend the diagram should name.

OpenTelemetryPlan/06-implementation-phases.md

@@ -484,7 +484,7 @@ graph LR
     end
 
     subgraph backends["Trace Backends"]
-        D["Jaeger / Tempo"]
+        D["Tempo"]
     end
 
     subgraph metrics["Metrics Stack"]