0.11 Migration: What to use instead of `CalculatedSize`

In bevy 0.10, `CalculatedSized` used to have a `size` field representing the node's "calculated" size, ie: size before layout constraints (is my understanding)

[This section of the migration guide](https://bevyengine.org/learn/migration-guides/0.10-0.11/#measurefunc-improvements) describe that "CalculatedSize has been renamed to ContentSize".

However, there is no mention of what to use instead of `size`! `ContentSize` has a `measure_func` field I've no idea how to get back the value that previously was there!

It seems `ContentSize` was changed in https://github.com/bevyengine/bevy/pull/7779, the migration guide mentions `UiImageSize`, but I also need to know the content size for text nodes.

Start with the migration guide's measurefunc-improvements section and review the ContentSize changes in PR 7779. Determine what replaced the former CalculatedSize.size value for text nodes, then update the guide to explain that migration path and verify that it addresses the reported gap.

Written by the indexing model from the issue text.