Update documentation to remove group boards from premium section

doc/README.md

@@ -88,7 +88,7 @@ Manage your [repositories](user/project/repository/index.md) from the UI (user i
 - [Discussions](user/discussions/index.md): Threads, comments, and resolvable discussions in issues, commits, and  merge requests.
 - [Issues](user/project/issues/index.md)
 - [Project issue Board](user/project/issue_board.md)
-- **(Premium)** [Group Issue Boards](user/project/issue_board.md#group-issue-boards)
+- [Group Issue Boards](user/project/issue_board.md#group-issue-boards)
 - **(Starter/Premium)** [Related Issues](user/project/issues/related_issues.md): create a relationship between issues
 - [Issues and merge requests templates](user/project/description_templates.md): Create templates for submitting new issues and merge requests.
 - [Labels](user/project/labels.md): Categorize your issues or merge requests based on descriptive titles.

doc/api/README.md

@@ -30,7 +30,7 @@ following locations:
 - [Group Members](members.md)
 - [Issues](issues.md)
 - [Issue Boards](boards.md)
-- **(Premium)** [Group Issue Boards] (group_boards.md)
+- [Group Issue Boards] (group_boards.md)
 - [Jobs](jobs.md)
 - [Keys](keys.md)
 - [Labels](labels.md)

doc/user/project/issue_board.md

@@ -304,7 +304,7 @@ If you don't have editing permission in a board, you're still able to see the co
 
 ## Group Issue Boards
 
-> Introduced in [GitLab Enterprise Edition 10.0 Premium](https://about.gitlab.com/2017/09/22/gitlab-10-0-released/#group-issue-boards).
+> Introduced in [GitLab Enterprise Edition 10.0](https://about.gitlab.com/2017/09/22/gitlab-10-0-released/#group-issue-boards).
 
 Group issue boards are analogous to project-level issue boards and they are accessible at the group
 navigation level. A group-level issue board allows you to view all issues from all projects in that group