Changing Flutter Candidate to 3.16.0-7.0.pre (#6509)

Automated Flutter candidate version bump.

flutter-candidate.txt

diff --git a/flutter-candidate.txt b/flutter-candidate.txt
index a3e360a..cd2d143 100644
--- a/flutter-candidate.txt
+++ b/flutter-candidate.txt
@@ -1 +1 @@
-3.16.0-5.0.pre
+3.16.0-7.0.pre