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/mattermost/main.go

@@ -6,18 +6,18 @@ package main
 import (
 	"os"
 
-	"github.com/mattermost/mattermost-server/server/v8/cmd/mattermost/commands"
+	"github.com/mattermost/mattermost/server/v8/cmd/mattermost/commands"
 	// Import and register app layer slash commands
-	_ "github.com/mattermost/mattermost-server/server/v8/channels/app/slashcommands"
+	_ "github.com/mattermost/mattermost/server/v8/channels/app/slashcommands"
 	// Plugins
-	_ "github.com/mattermost/mattermost-server/server/v8/channels/app/oauthproviders/gitlab"
+	_ "github.com/mattermost/mattermost/server/v8/channels/app/oauthproviders/gitlab"
 
 	// Enterprise Imports
-	_ "github.com/mattermost/mattermost-server/server/v8/channels/imports"
+	_ "github.com/mattermost/mattermost/server/v8/channels/imports"
 
 	// Blank imports for each product to register themselves
-	_ "github.com/mattermost/mattermost-server/server/v8/boards/product"
-	_ "github.com/mattermost/mattermost-server/server/v8/playbooks/product"
+	_ "github.com/mattermost/mattermost/server/v8/boards/product"
+	_ "github.com/mattermost/mattermost/server/v8/playbooks/product"
 )
 
 func main() {