`planetscale_database` with `default_branch` leads to inconsistent result after apply

[kota65535]

Creating the following resource:

resource "planetscale_database" "main" {
  organization         = "foo"
  name                 = "bar"
  cluster_size         = "PS-10"
  region               = "ap-northeast"
  default_branch       = "baz"
}

When I run terraform apply, got the following error.

╷
│ Error: Provider produced inconsistent result after apply
│ 
│ When applying changes to planetscale_database.main, provider "provider[\"registry.terraform.io/planetscale/planetscale\"]" produced an unexpected new value: .default_branch: was
│ cty.StringVal("baz"), but now cty.StringVal("main").
│ 
│ This is a bug in the provider, which should be reported in the provider's own issue tracker.
╵

This bug is similar to #135, but more reasonable because the default branch must exist before setting it as a default branch.
It would be better to deprecate default_branch input, and add a new resource planetscale_database_default_branch to resolve this.

For example:

# First we create a database
resource "planetscale_database" "main" {
  organization         = "foo"
  name                 = "bar"
  cluster_size         = "PS-10"
  region               = "ap-northeast"
}

# Then create another branch
resource "planetscale_branch" "dev" {
  organization  = planetscale_database.main.organization
  database      = planetscale_database.main.name
  parent_branch = "main"
  name          = "dev"
}

# Finally, set the another branch as default
resource planetscale_database_default_branch "dev" {
  database = planetscale_database.main.name
  branch = planetscale_branch.dev.name
}