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/scripts/config_generator/main.go

@@ -8,7 +8,7 @@ import (
 	"fmt"
 	"os"
 
-	"github.com/mattermost/mattermost-server/v6/model"
+	"github.com/mattermost/mattermost-server/server/v8/model"
 )
 
 // generateDefaultConfig writes default config to outputFile.

server/scripts/config_generator/main_test.go

@@ -8,7 +8,7 @@ import (
 	"os"
 	"testing"
 
-	"github.com/mattermost/mattermost-server/v6/model"
+	"github.com/mattermost/mattermost-server/server/v8/model"
 
 	"github.com/stretchr/testify/require"
 )