all: release v1.26.0

Change-Id: I15ecd5b768516fd2382cfbc8b337ad3d8817deab
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/302752
Trust: Joe Tsai <joetsai@digital-static.net>
Reviewed-by: Damien Neil <dneil@google.com>

internal/version/version.go

@@ -54,7 +54,7 @@ const (
 	Major      = 1
 	Minor      = 26
 	Patch      = 0
-	PreRelease = "rc.1.devel"
+	PreRelease = ""
 )
 
 // String formats the version string for this module in semver format.