In this commit, we update every in-tree go.mod to declare go 1.25 and
have CI build with 1.26.3 (Dockerfile uses the golang:1.26-alpine base).
Docs that mention the minimum required Go version are aligned with the
new floor as well.
Fixes#2527, which flagged the README.md / go.mod version mismatch (the
README claimed 1.22 while the root + v2transport go.mod files already
required 1.23.2).
The use of the GO111MODULE environment variable doesn't have any effect
anymore and hasn't for a couple of versions. The default was set to "on"
a while back, so we can remove that variable everywhere.
