Deprecate unused ButtonStyleButton.iconAlignment property (#160023)

Fixes [Deprecate unused `ButtonStyleButton.iconAlignment`
property](flutter/flutter#159782)

Refactor done in flutter/flutter#158503 makes
`ButtonStyleButton.iconAlignment` redundant. In this PR I'm marking it
as deprecated without migration guide. However, it may possible to
remove it altogether.


## Pre-launch Checklist

- [x] I read the [Contributor Guide] and followed the process outlined
there for submitting PRs.
- [x] I read the [Tree Hygiene] wiki page, which explains my
responsibilities.
- [x] I read and followed the [Flutter Style Guide], including [Features
we expect every widget to implement].
- [x] I signed the [CLA].
- [x] I listed at least one issue that this PR fixes in the description
above.
- [ ] I updated/added relevant documentation (doc comments with `///`).
- [ ] I added new tests to check the change I am making, or this PR is
[test-exempt].
- [ ] I followed the [breaking change policy] and added [Data Driven
Fixes] where supported.
- [ ] All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel
on [Discord].

<!-- Links -->
[Contributor Guide]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#overview
[Tree Hygiene]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md
[test-exempt]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#tests
[Flutter Style Guide]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Style-guide-for-Flutter-repo.md
[Features we expect every widget to implement]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Style-guide-for-Flutter-repo.md#features-we-expect-every-widget-to-implement
[CLA]: https://cla.developers.google.com/
[flutter/tests]: https://github.com/flutter/tests
[breaking change policy]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#handling-breaking-changes
[Discord]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Chat.md
[Data Driven Fixes]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Data-driven-Fixes.md

Author: TahaTesser

packages/flutter/lib/src/material/button_style_button.dart

@@ -86,6 +86,11 @@ abstract class ButtonStyleButton extends StatefulWidget {
     required this.clipBehavior,
     this.statesController,
     this.isSemanticButton = true,
+    @Deprecated(
+      'Remove this parameter as it is now ignored. '
+      'Use ButtonStyle.iconAlignment instead. '
+      'This feature was deprecated after v3.28.0-1.0.pre.',
+    )
     this.iconAlignment,
     this.tooltip,
     required this.child,
@@ -158,6 +163,11 @@ abstract class ButtonStyleButton extends StatefulWidget {
   final bool? isSemanticButton;
 
   /// {@macro flutter.material.ButtonStyleButton.iconAlignment}
+  @Deprecated(
+    'Remove this parameter as it is now ignored. '
+    'Use ButtonStyle.iconAlignment instead. '
+    'This feature was deprecated after v3.28.0-1.0.pre.',
+  )
   final IconAlignment? iconAlignment;
 
   /// Text that describes the action that will occur when the button is pressed or