Compatibility with go 1.21 and latest QUIC library

[anatol]

I am trying to compile syncthing with the latest version of the compiler/QUIC library and QUIC complains here

	conn, ok := c.(interface{ SetWriteBuffer(int) error })
	if !ok {
		return errors.New("connection doesn't allow setting of send buffer size. Not a *net.UDPConn?")
	}

as filteredConnObb does not have method SetWriteBuffer(int) error