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/plugintest/api.go

@@ -10,7 +10,7 @@ import (
 
 	mock "github.com/stretchr/testify/mock"
 
-	model "github.com/mattermost/mattermost-server/v5/model"
+	model "github.com/mattermost/mattermost-server/v6/model"
 )
 
 // API is an autogenerated mock type for the API type

plugin/plugintest/doc.go

@@ -7,5 +7,5 @@
 // https://godoc.org/github.com/stretchr/testify/mock
 //
 // If you need to import the mock package, you can import it with
-// "github.com/mattermost/mattermost-server/v5/plugin/plugintest/mock".
+// "github.com/mattermost/mattermost-server/v6/plugin/plugintest/mock".
 package plugintest

plugin/plugintest/driver.go

@@ -9,7 +9,7 @@ import (
 
 	mock "github.com/stretchr/testify/mock"
 
-	plugin "github.com/mattermost/mattermost-server/v5/plugin"
+	plugin "github.com/mattermost/mattermost-server/v6/plugin"
 )
 
 // Driver is an autogenerated mock type for the Driver type

plugin/plugintest/example_hello_user_test.go

@@ -13,9 +13,9 @@ import (
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
 
-	"github.com/mattermost/mattermost-server/v5/model"
-	"github.com/mattermost/mattermost-server/v5/plugin"
-	"github.com/mattermost/mattermost-server/v5/plugin/plugintest"
+	"github.com/mattermost/mattermost-server/v6/model"
+	"github.com/mattermost/mattermost-server/v6/plugin"
+	"github.com/mattermost/mattermost-server/v6/plugin/plugintest"
 )
 
 type HelloUserPlugin struct {

plugin/plugintest/helpers.go

@@ -5,8 +5,8 @@
 package plugintest
 
 import (
-	model "github.com/mattermost/mattermost-server/v5/model"
-	plugin "github.com/mattermost/mattermost-server/v5/plugin"
+	model "github.com/mattermost/mattermost-server/v6/model"
+	plugin "github.com/mattermost/mattermost-server/v6/plugin"
 	mock "github.com/stretchr/testify/mock"
 )
 

plugin/plugintest/hooks.go

@@ -10,9 +10,9 @@ import (
 
 	mock "github.com/stretchr/testify/mock"
 
-	model "github.com/mattermost/mattermost-server/v5/model"
+	model "github.com/mattermost/mattermost-server/v6/model"
 
-	plugin "github.com/mattermost/mattermost-server/v5/plugin"
+	plugin "github.com/mattermost/mattermost-server/v6/plugin"
 )
 
 // Hooks is an autogenerated mock type for the Hooks type