Changing Flutter Candidate to 3.16.0-0.0.pre (#6494)

Automated Flutter candidate version bump.

flutter-candidate.txt

diff --git a/flutter-candidate.txt b/flutter-candidate.txt
index 4cc5d48..46d964e 100644
--- a/flutter-candidate.txt
+++ b/flutter-candidate.txt
@@ -1 +1 @@
-3.15.0-10.0.pre
+3.16.0-0.0.pre