Changing Flutter Candidate to 3.20.0-12.0.pre (#7283)

Automated Flutter candidate version bump.

flutter-candidate.txt

diff --git a/flutter-candidate.txt b/flutter-candidate.txt
index ae35d86..227eb68 100644
--- a/flutter-candidate.txt
+++ b/flutter-candidate.txt
@@ -1 +1 @@
-3.20.0-10.0.pre
+3.20.0-12.0.pre