Previously, the original "404" template key was lost when "Default template" is merged into the available templates array, as the `array_merge()` function renumbers numeric keys. Later, when we merge templates on the client side, this caused duplicate "404" templates to appear in the dropdown.
By replacing the `array_merge()` call with `array_replace()`, we make sure to keep the original numeric keys.
Props Mamaduka, Toro_Unit, youknowriad.
Fixes#53898.
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