all: start post-v1.20.0 devel

Change-Id: I811fab64a8a22288760beef04021ded63bc4ea9b
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/221784
Reviewed-by: Herbie Ong <herbie@google.com>
diff --git a/runtime/protoimpl/version.go b/runtime/protoimpl/version.go
index 70d26ac..89990ce 100644
--- a/runtime/protoimpl/version.go
+++ b/runtime/protoimpl/version.go
@@ -53,7 +53,7 @@
 	versionMajor      = 1
 	versionMinor      = 20
 	versionPatch      = 0
-	versionPreRelease = ""
+	versionPreRelease = "devel"
 )
 
 // VersionString formats the version string for this module in semver format.