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
```

jobs/interfaces/active_users_interface.go

@@ -4,7 +4,7 @@
 package interfaces
 
 import (
-	"github.com/mattermost/mattermost-server/v5/model"
+	"github.com/mattermost/mattermost-server/v6/model"
 )
 
 type ActiveUsersJobInterface interface {

jobs/interfaces/expirynotify_interface.go

@@ -4,7 +4,7 @@
 package interfaces
 
 import (
-	"github.com/mattermost/mattermost-server/v5/model"
+	"github.com/mattermost/mattermost-server/v6/model"
 )
 
 type ExpiryNotifyJobInterface interface {

jobs/interfaces/export_delete_interface.go

@@ -3,7 +3,7 @@
 
 package interfaces
 
-import "github.com/mattermost/mattermost-server/v5/model"
+import "github.com/mattermost/mattermost-server/v6/model"
 
 type ExportDeleteInterface interface {
 	MakeWorker() model.Worker

jobs/interfaces/export_process_interface.go

@@ -4,7 +4,7 @@
 package interfaces
 
 import (
-	"github.com/mattermost/mattermost-server/v5/model"
+	"github.com/mattermost/mattermost-server/v6/model"
 )
 
 type ExportProcessInterface interface {

jobs/interfaces/import_delete_interface.go

@@ -3,7 +3,7 @@
 
 package interfaces
 
-import "github.com/mattermost/mattermost-server/v5/model"
+import "github.com/mattermost/mattermost-server/v6/model"
 
 type ImportDeleteInterface interface {
 	MakeWorker() model.Worker

jobs/interfaces/import_process_interface.go

@@ -4,7 +4,7 @@
 package interfaces
 
 import (
-	"github.com/mattermost/mattermost-server/v5/model"
+	"github.com/mattermost/mattermost-server/v6/model"
 )
 
 type ImportProcessInterface interface {

jobs/interfaces/indexer_interface.go

@@ -4,7 +4,7 @@
 package interfaces
 
 import (
-	"github.com/mattermost/mattermost-server/v5/model"
+	"github.com/mattermost/mattermost-server/v6/model"
 )
 
 type IndexerJobInterface interface {

jobs/interfaces/migrations_interface.go

@@ -4,7 +4,7 @@
 package interfaces
 
 import (
-	"github.com/mattermost/mattermost-server/v5/model"
+	"github.com/mattermost/mattermost-server/v6/model"
 )
 
 type MigrationsJobInterface interface {

jobs/interfaces/plugins_interface.go

@@ -4,7 +4,7 @@
 package interfaces
 
 import (
-	"github.com/mattermost/mattermost-server/v5/model"
+	"github.com/mattermost/mattermost-server/v6/model"
 )
 
 type PluginsJobInterface interface {

jobs/interfaces/product_notices_interface.go

@@ -4,7 +4,7 @@
 package interfaces
 
 import (
-	"github.com/mattermost/mattermost-server/v5/model"
+	"github.com/mattermost/mattermost-server/v6/model"
 )
 
 type ProductNoticesJobInterface interface {