Implement `ImageStyleGenerationTrait::generateImageUrl()` [#3539665]

Upstream in Flysystem, #2661588: Support delivering local image styles until remote upload is complete added an API for delivering image derivatives as soon as they are generated, and then they are copied to the remote adapter. Except, it didn't really work and needed fixed for non-local adapters at #3513799: Regression: Image style generation no longer works for non-local drivers. During D11 upgrade of this module, use of the new method was considered but it didn't work (because of the regression) and wasn't implemented. The newest RCs for Flysystem fix this, so now we can implement here and clear this deprecation.

Implement the new method.

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3539665-implement-imagestylegenerationtraitgenerateimageurl changes, plain diff MR !27
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deprecation-2.1.x changes, plain diff MR !28
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