Cover letters must always be either addressed specifically or neutrally.
By specifically, I mean using the actual hiring manager’s name if possible, e.g., “Dear Sandra Jones.”
By neutrally, I mean that one should use a general salutation, such as “Dear Hiring Manager” when the name of a hiring manager is not available.
“Dear Sir” or “To whom it may concern” invite speculation and possible rejection. What if the reader is a woman? And actually who is concerned? (Let us not go there.)
When in doubt, proceed with the conservative approach.