Standardize on use of "plan" and "state" variables

[mitchnielsen]

Summary

Context: https://github.com/PrefectHQ/terraform-provider-prefect/pull/193#discussion_r1626391852

Let's standardize on the use of plan and state for variable names when interacting with Terraform. For example:

func (r *BlockResource) Read(ctx context.Context, req resource.ReadRequest, resp *resource.ReadResponse) {
    var state BlockResourceModel
        // ...

This is how the Terraform provider tutorial names them, and (imo) is more intuitive to work with.

Acceptance criteria

• [ ] Variable names are all set to plan and state instead of model, etc.