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
```

app/plugin_shutdown_test.go

@@ -24,7 +24,7 @@ func TestPluginShutdownTest(t *testing.T) {
 			package main
 
 			import (
-				"github.com/mattermost/mattermost-server/v5/plugin"
+				"github.com/mattermost/mattermost-server/v6/plugin"
 			)
 
 			type MyPlugin struct {
@@ -39,7 +39,7 @@ func TestPluginShutdownTest(t *testing.T) {
 			package main
 
 			import (
-				"github.com/mattermost/mattermost-server/v5/plugin"
+				"github.com/mattermost/mattermost-server/v6/plugin"
 			)
 
 			type MyPlugin struct {