Remove net from list of banned gVisor packages.

`net` no longer requires `cgo`: https://pkg.go.dev/net#hdr-Name_Resolution

The `runsc` `go_binary` specifies `pure = True`, which would prevent it
from compiling if it required cgo.

PiperOrigin-RevId: 506722374

CONTRIBUTING.md

@@ -66,8 +66,6 @@ Rules:
 
     *   Itself.
     *   Go standard library.
-        *   Except (transitively) package "net", which would result in a cgo
-            binary. Use `//pkg/unet` instead.
     *   `@org_golang_x_sys//unix:go_default_library` (Go import
         `golang.org/x/sys/unix`).
     *   `@org_golang_x_time//rate:go_default_library` (Go import
@@ -86,6 +84,8 @@ Rules:
     *   `@com_github_opencontainers_runtime_spec//specs_go:go_default_library`
         (Go import `github.com/opencontainers/runtime-spec/specs_go`).
 
+*   For performance reasons, `runsc boot` may not run the `netpoller` goroutine.
+
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