Render icon for mobile navigation close button

[aduth]

🛠 Summary of changes

Updates rendering of the mobile navigation menu close button icon to use IconComponent instead of a custom SVG.

Why?

• This image comes from the Login.gov Design System, but is not used within the design system itself, so I'm aiming to remove it from there
• Avoids redundancy and reduces risk for desync in the styling of "Close" icons
• May marginally improve performance since the single sprite.svg SVG sprite image is assumed to be cached by the time this image is displayed

📜 Testing Plan

1. Go to http://localhost:3000 on a mobile device (or emulated browser viewport)
2. Sign in
3. From the account dashboard, click "Menu" in the top-right
4. Observe that the menu's close button is visible, shown as a blue "X" icon

👀 Screenshots

Visual differences are expected to be very minimal.

Before	After

[aduth]

• May marginally improve performance since the single sprite.svg SVG sprite image is assumed to be cached by the time this image is displayed

Thinking out loud a bit: This is probably true for this icon's usage, since it appears on the account page where we're using other icons. But I'm not sure the guidance holds true for replacing any ad-hoc icon with the sprite, since it may introduce the sprite download to a page where it doesn't already exist. For example, we have icons we could replace on the sign-in page (in the "Here's how you know" banner and language picker), but the sprite isn't currently being loaded on those pages, and it's about 22kb in size.

Some ideas:

• There might be opportunities to reduce the size of the sprite itself, e.g. updating outdated SVGO, or configure sprite generation to trim unnecessary data, or post-process the generated sprite with optimizing tool
• Bypass the sprite and render the original SVGs, optionally as inline elements