Change version to 1.55.0 (#6211)

Co-authored-by: Easwar Swaminathan <easwars@google.com>
diff --git a/version.go b/version.go
index 9948724..853ce0e 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 = "1.55.0"