Improve Settings page headings

### Description
There is currently some inconsistency with the headings on the settings page that I noticed when reviewing the SRP quiz. Semantic HTML could also be greatly improved.

https://github.com/MetaMask/metamask-extension/assets/8112138/4ea46233-c7ec-4021-b231-5feb071443ec

## Technical Details
- Improve the styling and the sematic html of the settings page
- Update "Settings" to ``
- Update "General", "Advanced", "Contacts", "Security & privacy", "Alerts", "Networks", "Experimental", "About" to be to use ``

## Acceptance Criteria
- heading styles are consistent and show visual priority on all settings pages
- html headings are in order e.g. h1, h2, h3 etc

Start by locating the settings page components and the existing Text usage for the Settings, General, Advanced, Contacts, Security & privacy, Alerts, Networks, Experimental, and About headings. Review how heading levels and styles are currently rendered, then verify that visual priority is consistent and the resulting HTML follows the required heading order across all settings pages.

Written by the indexing model from the issue text.