Text Changes: Clarify the Site Icon description wording

[R1shabh-Gupta]

Clarifies the Site Icon help text in Settings → General and the matching Customizer control. The wording now makes it clear that "square" refers to the aspect ratio, so rounded or circular icons are also valid.

Updated the same translatable string in both options-general.php and class-wp-customize-manager.php to keep them in sync. Placeholders are unchanged, and × is used to match Core's existing dimension format.

Trac ticket: https://core.trac.wordpress.org/ticket/65975

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