Add limit attribute to gallery shortcode

[Harshjadav08]

Description

Adds a limit attribute to the WordPress gallery shortcode.

The new attribute allows the gallery output to be limited to a specific number of attachments while preserving the existing default behavior with limit="-1".

Trac ticket: https://core.trac.wordpress.org/ticket/12799

Testing

• Ran PHPCS on src/wp-includes/media.php
• git diff --check passes
• Verified the gallery shortcode changes

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