Style: Use inter font size properties for heading sizes.

[ryelle]

This updates the headings across the site to use the correct "Inter Heading" levels, and with the parent-theme change, also shrinks the headings on small screens. This also cleans up some redundant settings and classes, and fixes the font-weight on the "Alternative text" label.

Fixes #24, See WordPress/wporg-parent-2021#164

Screenshots

Before	After

How to test the changes in this Pull Request:

1. Set the parent theme to use Theme.json: Add custom properties for Inter Heading font sizes wporg-parent-2021#164
2. Build the styles (in both repos)
3. Ensure the font sizes match the design
4. Shrink the browser to <600px, the heading sizes should drop down too.
5. In addition to the screenshots above, the headings on pages like Guidelines & photo submission should also change