datasource/gitlab_project_issues: Fix not_milestone to be simple string instead of list of strings

Author: timofurrer

docs/data-sources/project_issues.md

@@ -50,7 +50,7 @@ data "gitlab_project_issues" "all_with_foo" {
 - `not_assignee_id` (List of Number) Return issues that do not match the assignee id.
 - `not_author_id` (List of Number) Return issues that do not match the author id.
 - `not_labels` (List of String) Return issues that do not match the labels.
-- `not_milestone` (List of String) Return issues that do not match the milestone.
+- `not_milestone` (String) Return issues that do not match the milestone.
 - `not_my_reaction_emoji` (List of String) Return issues not reacted by the authenticated user by the given emoji.
 - `order_by` (String) Return issues ordered by. Valid values are `created_at`, `updated_at`, `priority`, `due_date`, `relative_position`, `label_priority`, `milestone_due`, `popularity`, `weight`. Default is created_at
 - `scope` (String) Return issues for the given scope. Valid values are `created_by_me`, `assigned_to_me`, `all`. Defaults to all.

internal/provider/data_source_gitlab_project_issues.go

@@ -137,8 +137,7 @@ var _ = registerDataSource("gitlab_project_issues", func() *schema.Resource {
 			},
 			"not_milestone": {
 				Description: "Return issues that do not match the milestone.",
-				Type:        schema.TypeList,
-				Elem:        &schema.Schema{Type: schema.TypeString},
+				Type:        schema.TypeString,
 				Optional:    true,
 			},
 			"my_reaction_emoji": {
@@ -251,7 +250,7 @@ func dataSourceGitlabProjectIssuesRead(ctx context.Context, d *schema.ResourceDa
 	}
 
 	if v, ok := d.GetOk("not_milestone"); ok {
-		options.NotMilestone = stringSetToStringSlice(v.(*schema.Set))
+		options.NotMilestone = gitlab.String(v.(string))
 	}
 
 	if v, ok := d.GetOk("scope"); ok {