v8.0 module release (#22975)

https://mattermost.atlassian.net/browse/MM-52079

```release-note
We upgrade the module version to 8.0. The new module path is github.com/mattermost-server/server/v8.
```


Co-authored-by: Doug Lauder <wiggin77@warpmail.net>

server/model/group_member.go

@@ -0,0 +1,23 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See LICENSE.txt for license information.
+
+package model
+
+import "net/http"
+
+type GroupMember struct {
+	GroupId  string `json:"group_id"`
+	UserId   string `json:"user_id"`
+	CreateAt int64  `json:"create_at"`
+	DeleteAt int64  `json:"delete_at"`
+}
+
+func (gm *GroupMember) IsValid() *AppError {
+	if !IsValidId(gm.GroupId) {
+		return NewAppError("GroupMember.IsValid", "model.group_member.group_id.app_error", nil, "", http.StatusBadRequest)
+	}
+	if !IsValidId(gm.UserId) {
+		return NewAppError("GroupMember.IsValid", "model.group_member.user_id.app_error", nil, "", http.StatusBadRequest)
+	}
+	return nil
+}