Change version to 1.56.0-dev (#6213)

diff --git a/version.go b/version.go
index 9948724..310603b 100644
--- a/version.go
+++ b/version.go
@@ -19,4 +19,4 @@
 package grpc
 
 // Version is the current grpc version.
-const Version = "1.55.0-dev"
+const Version = "1.56.0-dev"