Changing Flutter Candidate to 3.20.0-9.0.pre (#7242)

Automated Flutter candidate version bump.

flutter-candidate.txt

diff --git a/flutter-candidate.txt b/flutter-candidate.txt
index 8493657..da325c5 100644
--- a/flutter-candidate.txt
+++ b/flutter-candidate.txt
@@ -1 +1 @@
-3.20.0-6.0.pre
+3.20.0-9.0.pre