Remove autovalidate deprecations

[Piinks]

This removes the autovalidate parameter from

• DropdownButtonFormField
• TextFormField
• Form
• FormField

The replacement for which is autovalidateMode. This is supported by dart fix.

These deprecations have expired, as laid out in flutter.dev/go/deprecations-removed-after-2-5

Part of #90287

This affects the date picker and the time picker as well, but just under the hood. They were using restorable autovalidation through the old API.

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[flutter-dashboard]

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[Piinks]

This one will require a bit more work, it's being used in the time picker so I need to refactor that out.

Outdated

[Piinks]

Currently migrating internal customers.

[goderbauer]

LGTM

[Piinks]

Customers have been migrated, this can land now.