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/channels/app/slashcommands/command_code.go

@@ -6,10 +6,10 @@ package slashcommands
 import (
 	"strings"
 
-	"github.com/mattermost/mattermost-server/server/public/model"
-	"github.com/mattermost/mattermost-server/server/public/shared/i18n"
-	"github.com/mattermost/mattermost-server/server/v8/channels/app"
-	"github.com/mattermost/mattermost-server/server/v8/channels/app/request"
+	"github.com/mattermost/mattermost/server/public/model"
+	"github.com/mattermost/mattermost/server/public/shared/i18n"
+	"github.com/mattermost/mattermost/server/v8/channels/app"
+	"github.com/mattermost/mattermost/server/v8/channels/app/request"
 )
 
 type CodeProvider struct {