[CLOUDGA-25006] Deprecate root node info

[daniel-yb]

Deprecate root node info. Mark the root one optional.

Using the deprecated fields will now result in:

╷
│ Warning: Attribute Deprecated
│ 
│   with ybm_cluster.single_region,
│   on cluster-single-region.tf line 50, in resource "ybm_cluster" "single_region":
│   50:   node_config = {
│   51:     num_cores    = 2
│   52:     disk_size_gb = 10
│   53:   }
│ 
│ Remove this attribute's configuration as it's no longer in use and the attribute will be removed in the next major version of the provider. Please use cluster_region_info to specify node
│ config instead.
│ 
│ (and one more similar warning elsewhere)
╵

[posriniv]

Looks good to me!