Changing Flutter Candidate to 3.15.0-8.0.pre (#6470)

Automated Flutter candidate version bump.

flutter-candidate.txt

diff --git a/flutter-candidate.txt b/flutter-candidate.txt
index 0c178df..6f72b16 100644
--- a/flutter-candidate.txt
+++ b/flutter-candidate.txt
@@ -1 +1 @@
-3.15.0-5.0.pre
+3.15.0-8.0.pre