Remove and update outdated markup

Problem/Motivation

In reviewing the USWDS 2.x to 3.x documentation there is some outdated markup that persists in the most recent theme release.

Steps to reproduce

Proposed resolution

Update social media icons in the footer.

Remove unnecessary usa-social-link--[PLATFORM] CSS classes from templates/layout/page.html.twig
For the social media icons, these may have been left for backward compatibility as the rest of the markup appears to have been updated.

Update footer logo headings to use proper semantics.

Replace the h3 in this markup with a p for both the opening and closing tags in templates/layout/page.html.twig where class="usa-footer__logo-heading".

Remaining tasks

User interface changes

API changes

Data model changes