MM-53032: Fix module path after repo rename (#23689)

It was a good decision in hindsight to keep the public module as 0.x
because this would have been a breaking change again.

https://mattermost.atlassian.net/browse/MM-53032
```release-note
Changed the Go module path from github.com/mattermost/mattermost-server/server/v8 to github.com/mattermost/mattermost/server/v8.

For the public facing module, it's path is also changed from github.com/mattermost/mattermost-server/server/public to github.com/mattermost/mattermost/server/public
```

server/cmd/mmctl/mocks/client_mock.go

@@ -2,7 +2,7 @@
 // See LICENSE.txt for license information.
 
 // Code generated by MockGen. DO NOT EDIT.
-// Source: github.com/mattermost/mattermost-server/server/v8/cmd/mmctl/client (interfaces: Client)
+// Source: github.com/mattermost/mattermost/server/v8/cmd/mmctl/client (interfaces: Client)
 
 // Package mocks is a generated GoMock package.
 package mocks
@@ -14,7 +14,7 @@ import (
 	reflect "reflect"
 
 	gomock "github.com/golang/mock/gomock"
-	model "github.com/mattermost/mattermost-server/server/public/model"
+	model "github.com/mattermost/mattermost/server/public/model"
 )
 
 // MockClient is a mock of Client interface.