go.toolchain.rev: bump Go toolchain

* Go toolchain: https://github.com/tailscale/go/compare/186053ab2af2ef8fc43f3c1d1150216e89d4aff0...7ef7b403648ac3a9481a6e88e8b894eff50b0167

Triggered by @bradfitz via the bumpdep workflow.

Updates tailscale/corp#50253

Signed-off-by: Dep Updater <noreply+dep-updater@tailscale.com>
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