all: release v1.31.0

Change-Id: Ic67237749015f22a8657b65ad7250a6f95b742b4
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/506155
Reviewed-by: Michael Stapelberg <[email protected]>

Author: lfolger

internal/version/version.go

@@ -51,9 +51,9 @@ import (
 //  10. Send out the CL for review and submit it.
 const (
 	Major      = 1
-	Minor      = 30
+	Minor      = 31
 	Patch      = 0
-	PreRelease = "devel"
+	PreRelease = ""
 )
 
 // String formats the version string for this module in semver format.