Changing Flutter Candidate to 3.18.0-13.0.pre (#6988)

Automated Flutter candidate version bump.
diff --git a/flutter-candidate.txt b/flutter-candidate.txt
index 52bf860..ce0a549 100644
--- a/flutter-candidate.txt
+++ b/flutter-candidate.txt
@@ -1 +1 @@
-3.18.0-10.0.pre
+3.18.0-13.0.pre