Improve interaction with Recent Tasks/DAG Runs circles by ignoring pointer-events (#11786)

Resolves #11563. With the hover event triggering the tooltips, the tooltip was triggering a mouse-out event when the cursor was at the top of the circle (where the tooltip is presented). This was causing the fluttering behavior due to the looping off these events.

airflow/www/templates/airflow/dags.html

@@ -211,7 +211,7 @@
       </div>
     </div>
   </div>
-  <div id="svg-tooltip" class="tooltip top" style="position: fixed; display: none; opacity: 1">
+  <div id="svg-tooltip" class="tooltip top" style="position: fixed; display: none; opacity: 1; pointer-events: none;">
     <div class="tooltip-arrow"></div>
     <div class="tooltip-inner"></div>
   </div>
@@ -448,6 +448,7 @@
         .attr('vertical-align', 'middle')
         .attr('font-size', 8)
         .attr('y', 3)
+        .style('pointer-events', 'none')
         .text(function(d){ return d.count > 0 ? d.count : ''; });
     }
 
@@ -525,6 +526,7 @@
         .attr('vertical-align', 'middle')
         .attr('font-size', 8)
         .attr('y', 3)
+        .style('pointer-events', 'none')
         .text(function(d){ return d.count > 0 ? d.count : ''; });
     }