Make onChanged optional and add enabled to DropdownButtonFormField - #182419

Description of what this PR is changing or adding, and why:
Adds a breaking change page for the new `enabled` property logic on
`DropdownButton` and `DropdownButtonFormField`. This explains why the
`onChanged` callback is no longer required and clarifies the fallback
behavior for backward compatibility. It also guides developers on how to
properly separate interactive state (`enabled`) from widget behavior
(`onChanged`), highlighting what the `dart fix` covers and what requires
manual migration (like conditional logic).

Issues fixed by this PR (if any):
N/A but related to flutter/flutter#57953

PRs or commits this PR depends on (if any):
Depends on flutter/flutter#182419


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---------

Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com>
Co-authored-by: Parker Lougheed <[email protected]>

Introduced the `enabled` parameter in `DropdownButtonFormField`, allowing proper disabled/read-only usage without needing to pass dummy callbacks or remove existing ones.

Previously, the only way to disable a `DropdownButtonFormField` was to set `onChanged: null`. With this change, developers can explicitly control the interactive state via the `enabled` property. For example, you can set `enabled: false` while keeping `onChanged` defined, or set `enabled: true` to have an interactive field even if `onChanged` is omitted.

This change is fully backward compatible. If the `enabled` property is omitted (left as `null`), the field will fall back to the legacy behavior: it will be enabled if and only if `onChanged` is non-null.

Closes flutter/flutter#57953.

Port of flutter/flutter#182419.

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