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'Whom' because the pronoun is the object of 'concern'.

To whom it may concern.

In formal English grammar, who is subject case and whom is object case.

In this phrase, whom is used, because it is the object of the proposition to.

In colloquial English, the object form whom is being replaced by who. It is just one of the many ways our language is changing.

Let's look again, shall we?

to [whom it may concern]

to [whoever may read this letter]

The object of 'to' is the clause. The pronoun is governed by its role in the clause, which takes precedence over the preposition.

Mister MicawberThe object of 'to' is the clause. The pronoun is governed by its role in the clause, which takes precedence over the preposition.

Of course!