Fix Project Issue Boards API docs as ready for Core tier

Configurable parameters in issue board API (Assignee, Milestone,
Labels, Weight) are only available at Starter or higher.
Signed-off-by: Takuya Noguchi <takninnovationresearch@gmail.com>

doc/api/boards.md

@@ -171,7 +171,7 @@ Example response:
   }
 ```
 
-## Create an issue board **(STARTER)**
+## Create an issue board
 
 Creates a project issue board.
 
@@ -248,7 +248,7 @@ Example response:
   }
 ```
 
-## Update an issue board **(STARTER)**
+## Update an issue board
 
 > [Introduced](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/5954) in [GitLab Starter](https://about.gitlab.com/pricing/) 11.1.
 
@@ -258,15 +258,15 @@ Updates a project issue board.
 PUT /projects/:id/boards/:board_id
 ```
 
-| Attribute           | Type           | Required | Description |
-| ------------------- | -------------- | -------- | ----------- |
-| `id`                | integer/string | yes      | The ID or [URL-encoded path of the project](README.md#namespaced-path-encoding) owned by the authenticated user |
-| `board_id`          | integer        | yes      | The ID of a board |
-| `name`              | string         | no       | The new name of the board |
-| `assignee_id`       | integer        | no       | The assignee the board should be scoped to |
-| `milestone_id`      | integer        | no       | The milestone the board should be scoped to |
-| `labels`            | string         | no       | Comma-separated list of label names which the board should be scoped to |
-| `weight`            | integer        | no       | The weight range from 0 to 9, to which the board should be scoped to |
+| Attribute                    | Type           | Required | Description |
+| ---------------------------- | -------------- | -------- | ----------- |
+| `id`                         | integer/string | yes      | The ID or [URL-encoded path of the project](README.md#namespaced-path-encoding) owned by the authenticated user |
+| `board_id`                   | integer        | yes      | The ID of a board |
+| `name`                       | string         | no       | The new name of the board |
+| `assignee_id` **(STARTER)**  | integer        | no       | The assignee the board should be scoped to |
+| `milestone_id` **(STARTER)** | integer        | no       | The milestone the board should be scoped to |
+| `labels` **(STARTER)**       | string         | no       | Comma-separated list of label names which the board should be scoped to |
+| `weight` **(STARTER)**       | integer        | no       | The weight range from 0 to 9, to which the board should be scoped to |
 
 ```shell
 curl --request PUT --header "PRIVATE-TOKEN: <your_access_token>" "https://gitlab.example.com/api/v4/projects/5/boards/1?name=new_name&milestone_id=43&assignee_id=1&labels=Doing&weight=4"
@@ -329,7 +329,7 @@ Example response:
   }
 ```
 
-## Delete an issue board **(STARTER)**
+## Delete an issue board
 
 Deletes a project issue board.