all: start v1.20.1-devel

Change-Id: Ie2940cd41fd3c449b2233e1bc3b6ba5ba91038f2
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/222699
Reviewed-by: Damien Neil <dneil@google.com>
diff --git a/runtime/protoimpl/version.go b/runtime/protoimpl/version.go
index 5764b46..f98cf9d 100644
--- a/runtime/protoimpl/version.go
+++ b/runtime/protoimpl/version.go
@@ -53,7 +53,7 @@
 	versionMajor      = 1
 	versionMinor      = 20
 	versionPatch      = 1
-	versionPreRelease = ""
+	versionPreRelease = "devel"
 )
 
 // VersionString formats the version string for this module in semver format.