Update "Event Send Time Quantiles" graph to only use dots for the event persistence rate (#19399)

This is the same thing we already do in the [`matrix.org`
dashboard](https://grafana.matrix.org/d/000000012/synapse) and although
the purple dots aren't new (introduced in
https://github.com/matrix-org/synapse/pull/10001), you can see that was
the intention in https://github.com/element-hq/synapse/pull/18510. I
think this was just how our contrib dashboard looked at the time and
perhaps was a Grafana version mismatch thing which is why it didn't
translate.

changelog.d/19399.misc

@@ -0,0 +1 @@
+Update "Event Send Time Quantiles" graph to only use dots for the event persistence rate (Grafana dashboard).

contrib/grafana/synapse.json

@@ -416,6 +416,10 @@
               {
                 "id": "custom.axisPlacement",
                 "value": "right"
+              },
+              {
+                "id": "custom.fillOpacity",
+                "value": 0
               }
             ]
           },
@@ -443,6 +447,10 @@
               {
                 "id": "custom.axisPlacement",
                 "value": "right"
+              },
+              {
+                "id": "custom.fillOpacity",
+                "value": 0
               }
             ]
           }