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
```

api4/config.go

@@ -9,11 +9,11 @@ import (
 	"reflect"
 	"strings"
 
-	"github.com/mattermost/mattermost-server/v5/audit"
-	"github.com/mattermost/mattermost-server/v5/config"
-	"github.com/mattermost/mattermost-server/v5/model"
-	"github.com/mattermost/mattermost-server/v5/shared/mlog"
-	"github.com/mattermost/mattermost-server/v5/utils"
+	"github.com/mattermost/mattermost-server/v6/audit"
+	"github.com/mattermost/mattermost-server/v6/config"
+	"github.com/mattermost/mattermost-server/v6/model"
+	"github.com/mattermost/mattermost-server/v6/shared/mlog"
+	"github.com/mattermost/mattermost-server/v6/utils"
 )
 
 var writeFilter func(c *Context, structField reflect.StructField) bool