go.toolchain.branch: switch to Go 1.27

Promote the toolchain from Go 1.26.6 to Go 1.27.0, matching what
go.toolchain.next.rev has been testing. Besides the toolchain files
themselves (updated by pull-toolchain.sh), this bumps the go.mod go
directive, the Dockerfile golang base image, and the README, and
regenerates the depaware.txt files and the gzip assets in
tempfork/spf13/cobra and util/eventbus, whose bytes change with
Go 1.27's rewritten compress/flate.

Also bump golangci-lint to v2.13.1, the first release line built
with Go 1.27; the prebuilt v2.10.1 binary refuses to target a Go
version newer than the one it was built with.

Also bump golang.org/x/net to v0.58.0 (plus the sibling x/ module
upgrades it requires) to pick up upstream commit 8d10596d2624
(http2: avoid deadlocks in wrapped ClientConn state callback),
which we hit during Go 1.27 rc testing.

Also add docs/go-bump-checklist.md for next time.

Updates #20220

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
Change-Id: Ia3e4c9effafbc91227eed39efb52f1fba1b8d89c