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/featureflag/feature_flags_sync.go

@@ -13,8 +13,8 @@ import (
 	"github.com/splitio/go-client/v6/splitio/client"
 	"github.com/splitio/go-client/v6/splitio/conf"
 
-	"github.com/mattermost/mattermost-server/v5/model"
-	"github.com/mattermost/mattermost-server/v5/shared/mlog"
+	"github.com/mattermost/mattermost-server/v6/model"
+	"github.com/mattermost/mattermost-server/v6/shared/mlog"
 )
 
 type SyncParams struct {

app/featureflag/feature_flags_sync_test.go

@@ -8,7 +8,7 @@ import (
 
 	"github.com/stretchr/testify/require"
 
-	"github.com/mattermost/mattermost-server/v5/model"
+	"github.com/mattermost/mattermost-server/v6/model"
 )
 
 func TestGetStructFields(t *testing.T) {

app/featureflag/split_logger.go

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