Remove unnecessary stack field in gonet.UDPConn

[alanxone]

Description

I don't see the needs for putting stack.Stack in NewUDPConn, and either in UDPConn struct.

Ref: https://github.com/google/gvisor/blob/go/pkg/tcpip/adapters/gonet/gonet.go#L552

I propose following the design of NewTCPConn, remove stack and keep this input in DialUDP only, to make the API call consistent and easier understanding.

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