Improvement: Remove forest argument from adapt callback

[spenke91]

Closes #1279

Describe your changes here:

In line with #1279, this PR removes the forest argument, i.e., the uncommitted new forest, from the adapt callback arguments.

So far, this is just a draft: While the removal of the argument is already completed, #1279 additionally discussed the option of moderating this breaking change by using C++'s [[ deprecated ]] keyword. This is still to be done.

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