MM-36271: Bump major version to 6.0 (#17973)

https://mattermost.atlassian.net/browse/MM-36271

```release-note
We bump the major version to 6.0
```

plugin/interface_generator/main.go

@@ -373,8 +373,8 @@ import (
 	"net/http"
 	timePkg "time"
 
-	"github.com/mattermost/mattermost-server/v5/einterfaces"
-	"github.com/mattermost/mattermost-server/v5/model"
+	"github.com/mattermost/mattermost-server/v6/einterfaces"
+	"github.com/mattermost/mattermost-server/v6/model"
 )
 
 type apiTimerLayer struct {
@@ -415,8 +415,8 @@ import (
 	"net/http"
 	timePkg "time"
 
-	"github.com/mattermost/mattermost-server/v5/einterfaces"
-	"github.com/mattermost/mattermost-server/v5/model"
+	"github.com/mattermost/mattermost-server/v6/einterfaces"
+	"github.com/mattermost/mattermost-server/v6/model"
 )
 
 type hooksTimerLayer struct {
@@ -563,7 +563,7 @@ func generatePluginTimerLayer(info *PluginInterfaceInfo) {
 }
 
 func getPluginPackageDir() string {
-	dirs, err := goList("github.com/mattermost/mattermost-server/v5/plugin")
+	dirs, err := goList("github.com/mattermost/mattermost-server/v6/plugin")
 	if err != nil {
 		panic(err)
 	} else if len(dirs) != 1 {