Navigation: rename background color CSS class

[retrofox]

Description

This PR renames the CSS class from has-background-color to has-background when it defines a background color of the Navigation menu since, in general and by convention, has-background-color makes a reference to a color named background, in the same way that has-foreground-light-background-color, has-primary-background-color, ... does.
So the correct CSS class name to apply is has-background.

How has this been tested?

It has been tested with Maywood theme.

1. Install/Activate Maywood theme in your site
2. Create a Navigation menu.
3. Add links to the menu
4. Set the text and background color of the menu.

before
Confirm that the text color is not visualized either in the editor-canvas as well as in the front-end.

after
Confirm that the text color is rightly applied on both sides.

Screenshots

Types of changes

Checklist:

• My code is tested.
• My code follows the WordPress code style.

[marekhrabe]

Tested with Varia/Maywood theme and can confirm we are no longer hitting the bug with has-background-color.

This puts the Nav block in line with other blocks that were already using has-background as referenced here: https://github.com/WordPress/gutenberg/search?q=has-background&unscoped_q=has-background

Naming a color "background" is causing a strange confusion when those words get concatenated into the classname. Maybe we could come up with an advise to theme devs to avoid the name "background" and use something like "base" instead.

[retrofox]

Thanks @marekhrabe for your review. 👍