Update Material 3 progress indicators and slider guides for theme migration

[TahaTesser]

Fixes Update slider and progress indicator migration guides with year2023 flag from component themes

Presubmit checklist

• This PR is marked as draft with an explanation if not meant to land until a future stable release.
• This PR doesn’t contain automatically generated corrections (Grammarly or similar).
• This PR follows the Google Developer Documentation Style Guidelines — for example, it doesn’t use i.e. or e.g., and it avoids I and we (first person).
• This PR uses semantic line breaks of 80 characters or fewer.

[flutter-website-bot]

Visit the preview URL for this PR (updated for commit 4dea60c):

https://flutter-docs-prod--pr11733-update-m3-guides-fkcx5me7.web.app

[parlough]

Thanks for adding these clarifications for these design updates!

I have a question and a few small suggestions. I only made the suggestions for the first one, but they apply to each instance.

[parlough]

I tried this out and it's saying the year2023 parameter and corresponding property are deprecated?

[TahaTesser]

Yes, we added this flag with deprecation a few months ago to encourage users to read the migration steps and opt in the design.

[TahaTesser]

Eventually, we'll toggle the default value so users won't need to use the flag for the new design.

[parlough]

I see, thanks! It's a bit confusing though, as it's a deprecation warning, but in this case, you don't want to remove it.

Should we add an ignore comment to these snippets with an explanation of why the diagnostic is ignored?

[TahaTesser]

This is the deprecation message:

    @Deprecated(
      'Set this flag to false to opt into the 2024 progress indicator appearance. Defaults to true. '
      'In the future, this flag will default to false. Use ProgressIndicatorThemeData to customize individual properties. '
      'This feature was deprecated after v3.26.0-0.1.pre.',
    )
    this.year2023,

This message explains what will happen to the default value in the future and this flag isn't permanent hence the deprecation. Adding message to ignore will defeat the purpose of deprecation message.

[parlough]

Thanks for the PR and the adjustments @TahaTesser! Looks good to me.

Optionally do consider a comment or note like I mentioned in https://github.com/flutter/website/pull/11733/files#r1970471078.