#3213113 Customer Home: update card headings size and tags

Changes proposed in this Pull Request

• Update the card headings on Customer Home to be a smaller size. They were all fighting with the My Home page heading, especially on mobile. This also helps us move towards the explorations in #40734.
• Update the card headings to use a more semantic <h2> instead of <h1> all over the place.
• General cleanup of some other styles.
• Update headings to use title case and adding periods to subheadings per a recent decision.

@Automattic/dotcom-manage-design can you gut check me on the sizes here?

Before	After

Testing instructions

• Checkout the branch or use the calypso.live link
• Visit My Home to view updated headings
• Test with an established site with no checklist
• Test with a new site with checklist