go.mod: reorganize with Go 1.27's go mod tidy

Go 1.27's go mod tidy merges and reorganizes the require blocks once
the go directive in go.mod is 1.27 or newer. Land that mechanical
reorganization now (generated with a temporary 1.27 directive, with
the directive then restored) so the eventual toolchain switch doesn't
carry a 300+ line go.mod diff. Both Go 1.26's and Go 1.27's go mod
tidy leave the reorganized file unchanged, and go.sum and the selected
module versions are identical, so this is purely cosmetic.

Updates #20220

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
Change-Id: If7af51ca71ec30026034cf736f992e731c0e0531