Deprecate RendererInterface::render()'s sole $is_root_call parameter

Just ran into this exception and found the @todo via 📌 Try to replace path alias preloading with lazy generation Active and 📌 Create placeholders for more things Active - if you combine the two MRs from those issues and don't have bigpipe installed, you get the exception on various pages.

Converted the patch to an MR and dealt with conflicts/cs issues.

Lots of failures. Might be worth moving Wim's patch to a second MR and see where we land with that - looks like bc could be added quite straightforwardly to Wim's approach.