Fix flaky probing test.

This MidCallProbingRampupTriggeredByUpdatedBitrateConstraints blocks https://webrtc-review.googlesource.com/c/src/+/285740 submitting. I was able to complete the test locally, but cannot manage to do so remotely.

Bug: none
Change-Id: I75979af25552b4a31487a26e40857a713299e0eb
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/287022
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Commit-Queue: Diep Bui <diepbp@google.com>
Cr-Commit-Position: refs/heads/main@{#38848}

test/scenario/probing_test.cc

diff --git a/test/scenario/probing_test.cc b/test/scenario/probing_test.cc
index 74b68fc..86653ce 100644
--- a/test/scenario/probing_test.cc
+++ b/test/scenario/probing_test.cc
@@ -39,7 +39,7 @@
 
   const DataRate kStartRate = DataRate::KilobitsPerSec(300);
   const DataRate kConstrainedRate = DataRate::KilobitsPerSec(100);
-  const DataRate kHighRate = DataRate::KilobitsPerSec(2500);
+  const DataRate kHighRate = DataRate::KilobitsPerSec(1500);
 
   VideoStreamConfig video_config;
   video_config.encoder.codec =