Changing Flutter Candidate to 3.18.0-1.0.pre (#6894)

Automated Flutter candidate version bump.

flutter-candidate.txt

diff --git a/flutter-candidate.txt b/flutter-candidate.txt
index 5c336bd..f805202 100644
--- a/flutter-candidate.txt
+++ b/flutter-candidate.txt
@@ -1 +1 @@
-3.18.0-0.0.pre
+3.18.0-1.0.pre