Add index on internal notes

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/98435



Merged-by: Adam Hegyi <ahegyi@gitlab.com>
Approved-by: Andy Soiron <asoiron@gitlab.com>
Approved-by: Adam Hegyi <ahegyi@gitlab.com>
Co-authored-by: Nicolas Dular <ndular@gitlab.com>