update fusion migration guide callout about behavior change flags

### Contributions

- [x] I have read the contribution docs, and understand what's expected of me.

### Link to the page on docs.getdbt.com requiring updates

https://docs.getdbt.com/docs/dbt-versions/core-upgrade/upgrading-to-fusion?version=1.11

### What part(s) of the page would you like to see updated?

> All [behavior change flags](https://docs.getdbt.com/reference/global-configs/behavior-changes#behaviors) will be removed (generally enabled). You can no longer opt out of them using flags: in your dbt_project.yml.

We are actually preserving a few behavior change flags as "mature" instead of "removed" for fusion. We should adjust this language.

### Additional information

[Internal slack thread](https://dbt-labs.slack.com/archives/C0892L1G63X/p1776289541855449?thread_ts=1775593884.243399&cid=C0892L1G63X)