API Change default argument for `proportion` to be `True` in `tree.plot_tree`

### Summary

#27639 changed the default behavior of `tree_.value` to be proportions rather than absolute weighted sample counts. This then caused some discrepancies in how the tree is to be interested and plotted in the examples.

#29331 introduced a fix in the documentation to align the explanation of `tree_.value` and what was shown in the examples about the tree structure.

It is a bit weird that `plot_tree(tree)` by default shows different values compared to if you inspect `tree_.value` itself. 

### Proposal

1. Rather than go through a tedious deprecation cycle, I propose to change the default behavior of `plot_tree(tree)` to have `proportion=True` to match what is shown in `tree_.value`.

2. Ambitiously, I would also say that `tree_.value` is ambiguous and poorly named and we should also change it to `tree_.weighted_proportion`, which is more clear what that array is. Seeing as how `tree_.value` lives in Cython, and we still don't have public support of that API, I would even say that does not need a deprecation cycle, as the ppl who would use it are power users.

### Extra Comments
              @adam2392 if you feel adventurous you may want to open an issue about aggressively switching the default to `proportion=True` in `plot_tree` in order to match `tree.value` and get other maintainers feeling about it. Not sure if other visualization functions are affected by this to be 100% honest …

_Originally posted by @lesteve in https://github.com/scikit-learn/scikit-learn/issues/29331#issuecomment-2203040331_

COMMENT:
About 1., I would slightly lean towards switching to `proportion=True` by default without deprecation period, but I would agree this is a bit controversial. My main arguments:
- `plot_tree` is only useful for visualization/debugging. I don't think this can be used as some kind of intermediary results that are then used in further computations.
- the visual output of `plot_tree` would change but it is not that misleading since it goes from a count (integer) to something between 0 and 1.
- `tree_.value` has changed meaning in 1.4 (released January 18 2024) from weighted counts to weighted proportions. Noone complained until end April https://github.com/scikit-learn/scikit-learn/issues/28921 so maybe this is an indication that there are not that many people are relying on what `decision_tree.tree_.value` actually is ...

I would keep 2. for a separate PR because it seems more complicated and controversial, you can access `decision_tree.tree_.value` from Python as our [example](https://scikit-learn.org/dev/auto_examples/tree/plot_unveil_tree_structure.html#tree-structure) actually documents so straight renaming may break some code out there, even if as you said this is probably the case only for power users. There may be a not too complicated way to have the new name in the Cython data structure and a deprecated property `tree_.value` raising a `DeprecationWarning` and returning `tree._whatever_the_agreed_new_name_is`.

Edit: for 2. you could argue that `plot_tree` should show something with a proper name ("weighted counts" or "weighted proportions" depending on `proportion` argument) rather than just `value`.

Labels: RFC, module:tree