go.mod: update github.com/mdlayher/netlink to 1.6.0

This unbreaks some downstream users of tailscale who end up
with build errors from importing a v0 indirect dependency.

Signed-off-by: David Anderson <danderson@tailscale.com>
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Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f