Update Varsig prefix to Varsig v1

[expede]

Varsig v1.0 includes a varsig version segment. Also switched to Ed25519 because really no one should use RSA especially now that WebCrypto supports Ed25519

	Base64	Varsig Tag	Sig Config	Payload Codec
Before	NBIFEgEAcQ	V0: 0x34	RSA 2048 + SHA256	DAG-CBOR
After	NAHtAe0BE3E	V1: 0x34 0x01	Ed25519	DAG-CBOR

Also fixed some dead links that the linter was complaining about

[smoyer64]

New value matches the base64 encoded string encoded and decoded by go-varsig:

func TestUCANExampleV1(t *testing.T) {
    t.Parallel()

    // This test is the value shown in the UCAN v1.0.0 example, which is
    // an EdDSA varsig = v1 with the Ed25519 curve, SHA2_256 hashing and
    // DAG-CBOR content encoding.
    example, err := base64.RawStdEncoding.DecodeString("NAHtAe0BE3E")
    require.NoError(t, err)

    t.Run("Decode", func(t *testing.T) {
        t.Parallel()

        v, err := varsig.Decode(example)
        require.NoError(t, err)

        ed25519V, ok := v.(varsig.EdDSAVarsig)
        require.True(t, ok)

        assert.Equal(t, varsig.Version1, ed25519V.Version())
        assert.Equal(t, varsig.DiscriminatorEdDSA, ed25519V.Discriminator())
        assert.Equal(t, varsig.CurveEd25519, ed25519V.Curve())
        assert.Equal(t, varsig.HashAlgorithmSHA512, ed25519V.HashAlgorithm())
        assert.Equal(t, varsig.PayloadEncodingDAGCBOR, ed25519V.PayloadEncoding())
        assert.Len(t, ed25519V.Signature(), 0)
    })

    t.Run("Encode", func(t *testing.T) {
        t.Parallel()

        edDSAVarsig, err := varsig.NewEdDSAVarsig(
            varsig.CurveEd25519,
            varsig.HashAlgorithmSHA512,
            varsig.PayloadEncodingDAGCBOR,
        )
        require.NoError(t, err)

        assert.Equal(t, example, edDSAVarsig.Encode())
        t.Log(base64.RawStdEncoding.EncodeToString(edDSAVarsig.Encode()))
    })
}