protoc: update to the newest protobuf version (25.1)

Change-Id: I6b1c86eafe6eb515c2d718344717db0934386983
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/547255
Auto-Submit: Lasse Folger <lassefolger@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Stapelberg <stapelberg@google.com>

integration_test.go

@@ -35,7 +35,7 @@ var (
 	regenerate   = flag.Bool("regenerate", false, "regenerate files")
 	buildRelease = flag.Bool("buildRelease", false, "build release binaries")
 
-	protobufVersion = "25.0-rc2"
+	protobufVersion = "25.1"
 
 	golangVersions = func() []string {
 		// Version policy: same version as is in the x/ repos' go.mod.