internal/goversion: update Version to 1.26

Go 1.26 is in the process of being opened for development (to be
eventually released). This change marks the very beginning of its
development cycle, updating the Version value accordingly.

For #40705.
For #73829.

Change-Id: Ie4ce2d38160dd6283c08e10ecbd7d3a43ed92b48
Reviewed-on: https://go-review.googlesource.com/c/go/+/689877
Reviewed-by: Mark Freeman <mark@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

Author: mknyszek

src/internal/goversion/goversion.go

@@ -9,4 +9,4 @@ package goversion
 //
 // It should be updated at the start of each development cycle to be
 // the version of the next Go 1.x release. See go.dev/issue/40705.
-const Version = 25
+const Version = 26