fix: Update RPC struct Delegate

[huancheng-trili]

What

Update RPC struct Delegate according to the changelog.

Also re-grouped some old deprecated fields so that it's easier to identify which of them are no longer in use.

Test

Tested against ghostnet:

func main() {
	c, _ := rpc.NewClient("https://rpc.ghostnet.teztnets.com/", nil)
	ctx := context.TODO()
	after := tezos.MustParseBlockHash("BKm4PTHfr5saX1KyPDKiwVjSrfg7vrjFsABAtLUMKkAtbJWHZKo") // 10048212, v21
	before := tezos.MustParseBlockHash("BM6uDjxrzc6QusFYaHCQYd8ziJztZFUmSDQN2BdYqUXgCtE5Zm7") // 10047487, v20
        delegate := tezos.MustParseAddress("tz1NiaviJwtMbpEcNqSP6neeoBYj8Brb3QPv")
	b, e := c.GetDelegate(ctx, delegate, before)
	fmt.Println(e)
	fmt.Printf("%+v\n", b)

	b, e = c.GetDelegate(ctx, delegate, after)
	fmt.Println(e)
	fmt.Printf("%+v\n", b)
}

and did not receive any error.