all: start v1.28.1-devel

Change-Id: Ifa2563ae4661a99deaf015533d98413a38365797
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/419935
Reviewed-by: Michael Stapelberg <stapelberg@google.com>
diff --git a/internal/version/version.go b/internal/version/version.go
index b480c50..6c25e58 100644
--- a/internal/version/version.go
+++ b/internal/version/version.go
@@ -53,7 +53,7 @@
 	Major      = 1
 	Minor      = 28
 	Patch      = 1
-	PreRelease = ""
+	PreRelease = "devel"
 )
 
 // String formats the version string for this module in semver format.