telemetry: build the over-time charts from confirmed clusters (#10489)

Short-lived clusters report once under a fresh raft topology id and never
again, so CI runs and demo stacks each become their own cluster. Held
forward for the whole active window they pile up, and the volume server
line climbs every day while capacity stays flat.

Sum the fleet series over confirmed clusters only, the same set the
version and OS charts already use.

telemetry/server/dashboard/dashboard.go

@@ -155,11 +155,13 @@ func (h *Handler) ServeIndex(w http.ResponseWriter, r *http.Request) {
 
             <div class="chart-container">
                 <div class="chart-title">Volume Servers Over Time</div>
+                <div class="chart-subtitle">Confirmed clusters only</div>
                 <canvas id="serverChart" width="400" height="200"></canvas>
             </div>
 
             <div class="chart-container">
                 <div class="chart-title">Total Disk Usage Over Time</div>
+                <div class="chart-subtitle">Confirmed clusters only</div>
                 <canvas id="diskChart" width="400" height="200"></canvas>
             </div>