Add fixes for autovalidate deprecation in Form, FormField, TextFormField, and DropdownButtonFormField (#73996)

Small performance optimization by removing a duplicate setState call.

Enables custom text selection menus by allowing selectionControls to be passed to TextField et. al.

* migrate material to nullsafety

* address review comments

* address review comments

* fix build

https://github.com/flutter/flutter/issues/31931

This fixes a compile error in the form field example, and switches to using space instead of enter as the example for moving to the next field, since the text field on web unfocuses automatically when enter is pressed. (#61123)

* add language version 2.8 in packages/flutter

* enable non-nullable analyzer flag

This updates the Action API in accordance with the design doc for the changes: flutter.dev/go/actions-and-shortcuts-design-revision

Fixes #53276

smartDashesType and smartQuotesType params for text fields to control iOS's smart punctuation feature.

* implicit-casts:false in flutter/lib/src/material

* address review comments

* Update project.pbxproj files to say Flutter rather than Chromium

Also, the templates now have an empty organization so that we don't cause people to give their apps a Flutter copyright.

* Update the copyright notice checker to require a standard notice on all files

* Update copyrights on Dart files. (This was a mechanical commit.)

* Fix weird license headers on Dart files that deviate from our conventions; relicense Shrine.

Some were already marked "The Flutter Authors", not clear why. Their
dates have been normalized. Some were missing the blank line after the
license. Some were randomly different in trivial ways for no apparent
reason (e.g. missing the trailing period).

* Clean up the copyrights in non-Dart files. (Manual edits.)

Also, make sure templates don't have copyrights.

* Fix some more ORGANIZATIONNAMEs

initalValue should be initialValue

This flag controls Android's keyboard suggestions on/off

Use assertions to prevent multiline password fields from being created