Settings: Fix styles to align with rest of wp-admin

[obenland]

Fixes the display of update nags and brings the font size of the settings heading en par with other wp-admin screens.

Proposed changes:

• Adds margins for update nag.
• Adds heading styles for settings title.

Other information:

Before	After

Testing instructions:

• Visit the settings page on a WP install with a pending Core update.
• Make sure the nag has proper margins.
• Navigate to Privacy settings page.
• Make sure the title font sizes are the same between the two screens.

[pfefferle]

I can't even see the difference 🫣

[obenland]

It's subtle. If you open the screenshots in new tabs and switch between them, at least the banner should be easier to see.