Posts: Add disabled argument to wp_dropdown_categories() and wp_dropdown_pages()

[SainathPoojary]

This PR adds a disabled boolean argument to both wp_dropdown_categories() and wp_dropdown_pages(). When set to true, the generated <select> dropdowns are rendered with the HTML5 disabled attribute (<select disabled>).

Trac ticket: #20810

[github-actions]

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[github-actions]

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Props sainathpoojary.

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