Replace deprecated USWDS v1 form hint class

[aduth]

Description of Changes:

Replaces .usa-form-hint with .usa-hint.

.usa-form-hint is a legacy class name from USWDS v1, replaced in USWDS v2 with .usa-hint (documentation).

We still define custom styles for this class in the Login.gov Design System (source), but I'd like to remove these styles, and this appears to the only instance in use I could find.

This does have a visual impact, but it is intended to bring into alignment with hints on other Login.gov sites. If needed, we could add additional utility classes to restore the original style.

Effective styles:

Before:

.usa-form-hint {
    padding-top: 0.25rem;
    padding-bottom: 0.25rem;
    display: inline-block;
}

After:

.usa-hint {
    color: #767676;
}

Screenshots:

Before	After