Improve visual styling of mobile menu toggle when menu is open

[techfg]

Description

• Closes Improve visual indication of difference between open and close menu #2699
• What does this PR change? Changes the icon used when the menu is open to close ("X") to improve the visual indication that the behavior of clicking the button is different. The current approach is to change the background color and drop the box shadow, however darkening the button gives the appearance that the button is disabled and dropping the box shadow is difficult to discern in dark theme. The state of the button is not changing when the menu opens/closes, only its behavior is which is more commonly represented with a change in icon vs. a change in style.
• Did you change something visual? A before/after screenshot can be helpful.

Dark Theme

	Open	Closed
Current
Proposed

Light Theme

	Open	Closed
Current
Proposed

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🦋 Changeset detected

Latest commit: 2d69f6f

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@astrojs/starlight	Patch

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