Block Editor: URLInput: Remove autoFocus as a default prop of URLInput

[aduth]

Previously: #18061 (comment)

Currently, URLInput assigns auto-focus to its rendered input. Auto-focus is largely discouraged (we even enforce a rule) and it should not be the default behavior for such a basic component, as it limits (complicates) reusability and makes unnecessary assumptions about its rendering context. It's assumed that this was the default largely based on the assumption that the input is typically rendered within a Popover, which is not something we should be assuming.

Task: Avoid assigning autoFocus in URLInput. It should still be supported as an optional (opt-in) prop in cases where it is appropriate. This usage and associated cautions should be sufficiently documented.

[youknowriad]

this has been done recently.