Simplify the global navigation markup

[NicolaSaunders]

Describe the issue
As reported by @gosko, the markup required for the global navigation is quite verbose and there is a preference to try and simplify it. The main reasons for the verbosity are:

• the use of the .clean-list class on lists to remove all bullets/margin/padding
• adding role="list" to the lists to ensure that when list styles are removed by the .clean-list class, WebKit browsers will continue to correctly honour the list semantics
• inline SVGs used for decorative icons

Recommended solution

• Remove default bullets/margin/padding on lists used within <nav> elements, thereby removing the need to apply the .clean-list class
• Drop the requirement to add role="list" to lists used within <nav> elements, as per the Jan 2023 update to the "Fixing" lists blog post
• Replace inline SVG icons with pseudo elements styled with CSS

Additional context
Removing default bullets/margin/padding on lists used within <nav> elements means that the use of the .clean-list class and role="list" are no longer needed in other areas too. For example, see issue #669

[NicolaSaunders]

Proposed solution is at w3c/w3c-website-templates-bundle#150