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/boards/ws/mocks/mockpluginapi.go

@@ -2,7 +2,7 @@
 // See LICENSE.txt for license information.
 
 // Code generated by MockGen. DO NOT EDIT.
-// Source: github.com/mattermost/mattermost-server/v6/plugin (interfaces: API)
+// Source: github.com/mattermost/mattermost-server/server/v8/plugin (interfaces: API)
 
 // Package mocks is a generated GoMock package.
 package mocks
@@ -13,7 +13,7 @@ import (
 	reflect "reflect"
 
 	gomock "github.com/golang/mock/gomock"
-	model "github.com/mattermost/mattermost-server/v6/model"
+	model "github.com/mattermost/mattermost-server/server/v8/model"
 )
 
 // MockAPI is a mock of API interface.