all: release v1.28.1

Change-Id: I2987633792a1429079cfebdf4c03d5629e4a448a
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/419934
Reviewed-by: Michael Stapelberg <stapelberg@google.com>

Author: lfolger

internal/version/version.go

@@ -52,8 +52,8 @@ import (
 const (
 	Major      = 1
 	Minor      = 28
-	Patch      = 0
-	PreRelease = "devel"
+	Patch      = 1
+	PreRelease = ""
 )
 
 // String formats the version string for this module in semver format.