Deprecate Group Field of Instance Options; Retype Group in InstanceUpdateOptions to be String Pointer

[zliang-akamai]

📝 Description

Users may want to delete the value of group, a deprecated attribute of a Linode instance. We can make it to be a pointer to allow users pass in an empty string to remove the attribute on the cloud.
Original issue is reported in our TF provider repository: linode/terraform-provider-linode#1296

✔️ How to Test

Automated Testing

make ARGS="-run TestInstance" fixtures

Manual Testing

linodeClient := linodego.NewClient(oauth2Client)
linodeClient.SetDebug(true)
instance, _ := linodeClient.CreateInstance(context.Background(), linodego.InstanceCreateOptions{
	Label: "test-group-remove2",
	Type: "g6-nanode-1",
	Image: "linode/debian12",
	Group: "test-group",
	Region: "us-mia",
	RootPass: "wevgfhyuwe23uifcgbvwuiovfcbujwosbv",
})
group := ""
instance, _ = linodeClient.UpdateInstance(
	context.Background(),
	instance.ID,
	linodego.InstanceUpdateOptions{
		Group: &group,
	},
)

fmt.Printf("instance group: '%s'", instance.Group)

Expected result: instance group: ''

[lgarber-akamai]

LGTM, nice work! We'll need to make sure we communicate that this is a breaking change in the release notes given how wide-reaching it is.

[yec-akamai]

Nice work! Tests passed locally