Update heading styles

[gabydisarli]

Issue description

We use different heading styles across the app which leads to a disjointed experience for our users. There has been lots of customized heading classes over the course of the project which is hard to manage on the design and dev side. We should have a family of heading styles that can be used across the request form and domain management pages.

Both design and dev should work together to find all heading differences and determine a resolution.

Acceptance criteria

• Dev and Designer audit the code for all existing classes of H1,H2,H3,etc
• Dev, work with a designer to unify heading styles to follow Figma design system
• Dev: changes to any heading/text changes desired by the designer are considered in scope and changed
• Dev: if applicable, refactor areas of our scss using/defining these headers if deemed necessary.

Additional context

Note: These notes are relevant to our H3 style only.
We override the h3 in several areas. The screenshots are of the Senior official page, the domain overview page, and domain request page. There are other areas in the code where we are overwriting h3 that are not pictured here.
Align h3 class to have these properties

• font size: 1.2 em
• line height: 1.15
• font weight: 600 semi-bold
  See slack thread for other areas where we are overriding h3.

[gabydisarli]

Rachid's PR is up and its conditionally approved with some minor changes. See audit spreadsheet for more detail on our final heading decisions.