all: release v1.21.0

Change-Id: I6a91f3efd45ab0a446b220fddc8bd03aec93b388
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/228177
Reviewed-by: Damien Neil <dneil@google.com>
diff --git a/internal/version/version.go b/internal/version/version.go
index 03f0aab..cd84284 100644
--- a/internal/version/version.go
+++ b/internal/version/version.go
@@ -52,9 +52,9 @@
 //	10. Send out the CL for review and submit it.
 const (
 	Major      = 1
-	Minor      = 20
-	Patch      = 1
-	PreRelease = "devel"
+	Minor      = 21
+	Patch      = 0
+	PreRelease = ""
 )
 
 // String formats the version string for this module in semver format.