all: release v1.20.0

Change-Id: I3bc9d46f836535393059b9a73e8364211eee9e38
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/221783
Reviewed-by: Herbie Ong <herbie@google.com>

runtime/protoimpl/version.go

@@ -53,7 +53,7 @@ const (
 	versionMajor      = 1
 	versionMinor      = 20
 	versionPatch      = 0
-	versionPreRelease = "rc.1.devel"
+	versionPreRelease = ""
 )
 
 // VersionString formats the version string for this module in semver format.