Block Editor: URLInput: Remove "required" as a default prop of URLInput

[aduth]

Previously: #18061 (comment)

Currently, URLInput assigns required attribute to its rendered input. This limits its reusability and makes unnecessary assumptions about its rendering context. It's assumed that this was the default largely based on the assumption of specific contexts in which it was rendered (i.e. RichText link insertion).

Task: Avoid assigning required 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.