go.mod: bump x/net to 0.17 for CVE-2023-39325

https://go.googlesource.com/net/+/b225e7ca6dde1ef5a5ae5ce922861bda011cfabd

Updates tailscale/corp#15165

Change-Id: Ia8b5e16b1acfe1b2400d321034b41370396f70e2
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