go.mod: bump x/net for http2-in-std fix for Go 1.27

There's a deadlock in x/net/http2 with Go 1.27 that's
fixed with v0.58. This is prep for switching to Go 1.27.

Updates #20220

Change-Id: I8e81eafc0c2f78e1760b36e9d39ac4f6046565af
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>