feat: allow updating a project's instance size

[savme]

What kind of change does this PR introduce?

feature

What is the current behavior?

Changing instance_size is not supported.
Attempting to do so results in the following error: Update is not supported for project resource: <project>

What is the new behavior?

Plans like this are supported

  ~ resource "supabase_project" "test" {
        id                = "xyz"
      ~ instance_size     = "micro" -> "small"
        name              = "test"
        # (3 unchanged attributes hidden)
    }