When themes register default headers, they can use sprintf style placeholder
strings in place of the template directory URI, which WordPress transforms
in several places by running the value of
`get_theme_support( 'custom-header', 'default-image' )` through `sprintf()`.
This fixes a few places where WordPress skipped the `sprintf()` step and
referenced the `get_theme_support()` value directly.
Props bradyvercher for initial patch.
See #38557.
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