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/message_export.go

@@ -0,0 +1,50 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See LICENSE.txt for license information.
+
+package model
+
+import "encoding/json"
+
+type MessageExport struct {
+	TeamId          *string
+	TeamName        *string
+	TeamDisplayName *string
+
+	ChannelId          *string
+	ChannelName        *string
+	ChannelDisplayName *string
+	ChannelType        *ChannelType
+
+	UserId    *string
+	UserEmail *string
+	Username  *string
+	IsBot     bool
+
+	PostId         *string
+	PostCreateAt   *int64
+	PostUpdateAt   *int64
+	PostDeleteAt   *int64
+	PostMessage    *string
+	PostType       *string
+	PostRootId     *string
+	PostProps      *string
+	PostOriginalId *string
+	PostFileIds    StringArray
+}
+
+type MessageExportCursor struct {
+	LastPostUpdateAt int64
+	LastPostId       string
+}
+
+// PreviewID returns the value of the post's previewed_post prop, if present, or an empty string.
+func (m *MessageExport) PreviewID() string {
+	var previewID string
+	props := map[string]any{}
+	if m.PostProps != nil && json.Unmarshal([]byte(*m.PostProps), &props) == nil {
+		if val, ok := props[PostPropsPreviewedPost]; ok {
+			previewID = val.(string)
+		}
+	}
+	return previewID
+}