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
```

shared/filestore/filesstore_test.go

@@ -16,7 +16,7 @@ import (
 	"github.com/stretchr/testify/suite"
 	"github.com/xtgo/uuid"
 
-	"github.com/mattermost/mattermost-server/v5/shared/mlog"
+	"github.com/mattermost/mattermost-server/v6/shared/mlog"
 )
 
 func randomString() string {

shared/filestore/localstore.go

@@ -13,7 +13,7 @@ import (
 
 	"github.com/pkg/errors"
 
-	"github.com/mattermost/mattermost-server/v5/shared/mlog"
+	"github.com/mattermost/mattermost-server/v6/shared/mlog"
 )
 
 const (

shared/filestore/mocks/FileBackend.go

@@ -7,7 +7,7 @@ package mocks
 import (
 	io "io"
 
-	filestore "github.com/mattermost/mattermost-server/v5/shared/filestore"
+	filestore "github.com/mattermost/mattermost-server/v6/shared/filestore"
 
 	mock "github.com/stretchr/testify/mock"
 

shared/filestore/s3store.go

@@ -17,7 +17,7 @@ import (
 	"github.com/minio/minio-go/v7/pkg/encrypt"
 	"github.com/pkg/errors"
 
-	"github.com/mattermost/mattermost-server/v5/shared/mlog"
+	"github.com/mattermost/mattermost-server/v6/shared/mlog"
 )
 
 // S3FileBackend contains all necessary information to communicate with