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/cmd/mattermost/main.go

@@ -6,14 +6,14 @@ package main
 import (
 	"os"
 
-	"github.com/mattermost/mattermost-server/v6/server/cmd/mattermost/commands"
+	"github.com/mattermost/mattermost-server/server/v8/cmd/mattermost/commands"
 	// Import and register app layer slash commands
-	_ "github.com/mattermost/mattermost-server/v6/server/channels/app/slashcommands"
+	_ "github.com/mattermost/mattermost-server/server/v8/channels/app/slashcommands"
 	// Plugins
-	_ "github.com/mattermost/mattermost-server/v6/model/oauthproviders/gitlab"
+	_ "github.com/mattermost/mattermost-server/server/v8/model/oauthproviders/gitlab"
 
 	// Enterprise Imports
-	_ "github.com/mattermost/mattermost-server/v6/server/channels/imports"
+	_ "github.com/mattermost/mattermost-server/server/v8/channels/imports"
 )
 
 func main() {