all: start v1.21.0-devel

Change-Id: Ib9df04de7a3f46f3a3b512e92cc7694ec0f4f927
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/228178
Reviewed-by: Damien Neil <dneil@google.com>
diff --git a/internal/version/version.go b/internal/version/version.go
index cd84284..22d54f6 100644
--- a/internal/version/version.go
+++ b/internal/version/version.go
@@ -54,7 +54,7 @@
 	Major      = 1
 	Minor      = 21
 	Patch      = 0
-	PreRelease = ""
+	PreRelease = "devel"
 )
 
 // String formats the version string for this module in semver format.