go.mod: bump inet.af/netstack

Updates #2642 (I'd hoped, but doesn't seem to fix it)

Change-Id: Id54af7c90a1206bc7018215957e20e954782b911
Signed-off-by: Brad Fitzpatrick <bradfitz@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