Remove new forest from adapt callback arguments

[lukasdreyer]

Feature request

Remove non committed new forest from adapt callback, see:

t8code/src/t8_forest/t8_forest_general.h

Lines 112 to 113 in facc24c

	/* TODO: Do we really need the forest argument? Since the forest is not committed yet it
	* seems dangerous to expose to the user. */

Estimated priority
Which of these is most applicable (remove the others):

"Priority: medium" Should be solved within half a year

Additional context
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[dutkalex]

Hi! This is indeed a good idea IMO, as the current API is really confusing and I really can't think of a valid usecase for querying the not-yet-commited forest.
I would just like to advocate for this breaking change to be handled through an explicit deprecation process to give users a little bit of time and flexibility. I don't know your policy regarding C++ standards, but if you could ship with both the new API (without the not-yet-commited forest) and the old one, with a [[ deprecated ( "please migrate to the new API" ) ]] attribute (which requires C++14 IIRC), that would be great :)