[PP-1]Allow RenderableInterface objects to be passed as the $elements parameter in RendererInterface::render* methods

I love this, deprecation might be premature though, removing render arrays needs to be very very cautious I think.

In this context, deprecation means notifying that the union type declaration is coming in D12, not that arrays will be removed.

Ah that makes more sense.

Not yet, feel free to create one if you get to it before me.

Got the MR started with an updated test. Surprisingly everything passes.
Leaving at NR for some clean up.

OK, I think the MR is ready for now.

A couple notes:

• I didn't change any of the protected methods in Renderer, to keep the scope of the changes down. The render objects are converted back to arrays before the calls to the protected methods
• Not sure if the type declaration announcement changes are done completely or if there are more steps to do the deprecations
• Also not sure about how to do the change record. Should https://www.drupal.org/node/3534020 → be reused and updated? But then, we can't make changes to it until this issue is merged? How do we handle superseding changes to the same method signature?
• TIL (well, yesterday, technically) ternaries and arrow functions don't really support reference variables

The Needs Review Queue Bot → tested this issue. It no longer applies to Drupal core. Therefore, this issue status is now "Needs work".

This does not mean that the patch necessarily needs to be re-rolled or the MR rebased. Read the Issue Summary, the issue tags and the latest discussion here to determine what needs to be done.

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Since 📌 Slowly, very slowly start OOPifying the render system Needs review has been reverted for now, postponing.