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
```

services/docextractor/combine.go

@@ -6,7 +6,7 @@ package docextractor
 import (
 	"io"
 
-	"github.com/mattermost/mattermost-server/v5/shared/mlog"
+	"github.com/mattermost/mattermost-server/v6/shared/mlog"
 )
 
 type combineExtractor struct {

services/docextractor/docextractor_test.go

@@ -13,7 +13,7 @@ import (
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
 
-	"github.com/mattermost/mattermost-server/v5/utils/testutils"
+	"github.com/mattermost/mattermost-server/v6/utils/testutils"
 )
 
 func TestExtract(t *testing.T) {

services/docextractor/pdf_test.go

@@ -7,7 +7,7 @@ import (
 	"bytes"
 	"testing"
 
-	"github.com/mattermost/mattermost-server/v5/utils/testutils"
+	"github.com/mattermost/mattermost-server/v6/utils/testutils"
 	"github.com/stretchr/testify/require"
 )