go.toolchain.next.{branch,rev}: start testing Go 1.27 (go1.27rc2)

Point the TS_GO_NEXT=1 toolchain at the tailscale.go1.27 branch so we
can start testing Go 1.27 ahead of its release.

Updates #20220

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
Change-Id: I7947f1e79b713ff84e0418aef4ed3b17f7fbdbd5
This commit is contained in:
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Gitea Version: 1.28.0+dev-477-g8b6ad49a5f