I18N: In `wp_dropdown_languages()` rename the new `show_site_locale_default`...

I18N: In `wp_dropdown_languages()` rename the new `show_site_locale_default` argument to `show_option_site_default`.

This makes it consistent with arguments of other `wp_dropdown_*()` functions.

Props SergeyBiryukov.
See #38632.
Fixes #38871.
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