I noticed that there are helper migration functions that apply settings
transformations to multiple layers (root, platform, release-channel, and
profile):

`migrate_settings`
`migrate_language_setting`

Both of these handle the old settings profile forms, so I've updated
them to handle either form, for usage in historical and future
migrations.

Migration `m_2026_04_10` actually introduced a wrapper function to
provide to `migrate_settings` to handle both forms, which is what clued
me into the fact that I missed these migration helpers when I wrote the
`m_2026_04_01` originally (I didn't know they existed).

Self-Review Checklist:

- [X] I've reviewed my own diff for quality, security, and reliability
- [N/A] Unsafe blocks (if any) have justifying comments
- [N/A] The content is consistent with the [UI/UX
checklist](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)
- [X] Tests cover the new/changed behavior
- [X] Performance impact has been considered and is acceptable

Closes #ISSUE

Release Notes:

- N/A

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Co-authored-by: Smit Barmase <heysmitbarmase@gmail.com>