Fix navigation wrapper classname duplication - #78971

What?

Fixes #62690.

Alternative to #63801 that goes a bit further.

This PR (along with the already merged #77419) should allow responsive typography to work for the navigation block.

Why?

In trunk, the navigation block's wrapper class names and styles are incorrectly duplicated on both the top level <nav> element and the <ul> element that's a child of <nav>:

<nav class="wp-block-navigation is-layout-flex wp-block-navigation-is-layout-flex">
  <ul class="wp-block-navigation__container wp-block-navigation">

When responsive styles are added, they're done so through a wp-states class only on the nav wrapper:

<nav class="wp-block-navigation is-layout-flex wp-block-navigation-is-layout-flex wp-states-2e5ab777">
  <ul class="wp-block-navigation__container wp-block-navigation">

The problem is that the wp-block-navigation class has a style rule that sets the default font size for the navigation block:

:root :where(.wp-block-navigation) {
    font-size: var(--wp--preset--font-size--medium);
}

And it overrules any responsive font size applied via the wp-states, preventing responsive font styles from working correctly. This PR should resolve the issue without breaking any nav block styles.

How?

There are a few options to solve this:
A. Remove the duplicated wp-block-navigation from the ul (what this PR does)
B. Also duplicate the wp-states class to the inner ul. (essentially double down on the bug)
C. Make the default font size class only apply to the wrapper (update the selector to :root :where(nav.wp-block-navigation).

I chose A because B & C are essentially doubling down on buggy behavior, introducing more complexity.

The main drawback of A is that it will be more likely to cause back compat issues if a theme relies on those classnames. A dev note is definitely needed.

Testing Instructions

This will need a lot of general testing to make sure the style features in the nav block still work. I believe they do, but I may be missing something. I can see that the second bug mentioned in #68350 is still present, it causes inconsistencies for submenus between the editor and frontend, but wasn't introduced in this PR.

1. Add a nav block that has some links and submenus
2. Toggle on the tablet state from dropdown on the block card and select.
3. Set font styles for the tablet state only, check that the styles apply correctly in the editor
4. Switch back to desktop and check that the styles aren't applied
5. Save and view the frontend
6. Resize the viewport between desktop / tablet / mobile, and check that the font styles are applied correctly.

Screenshots

Use of AI Tools

Codex / OpenCode