The deprecated `[RawScrollbar/Scrollbar/CupertinoScrollbar/ScrollbarThemeData].isAlwaysShown` has expired and is removed in thi PR.
Also, `Scrollbar.hoverThickness` is also removed after having expired.

The replacement for `isAlwaysShown` is `thumbVisibility`. This deprecation was introduced in #96957. The name change came from new behaviors like a track that is conditionally visible. Since the original API only referred to the thumb, the name was changed to reflect this intention. This change is supported by dart fix. ✅ 

The replacement for `hoverThickness` is `ScrollbarThemeData.thickness`, which is a MaterialStateProperty that allows for the thickness to respond to multiple states including hover. This deprecation was introduced in #97173. This change is not supported by dart fix. 😞  

Part of https://github.com/flutter/flutter/issues/127042

Remove 1745 decorative breaks

* Use `curly_braces_in_flow_control_structures` for `material`

* include test/material

* add back removed comments

This does a cleanup of the examples, removing all of the "section" markers and extra comments that we don't need anymore now that the samples are no longer in the source code. It also removes the --template arguments from the {@tool dartpad} and {@tool sample} directives, since those are no longer used. It converts two examples that I discovered were still embedded into linked examples in the examples folder.

I didn't delete the templates from the snippets config folder yet, because there are still embedded samples in the dart:ui package from the engine that use them. Once dart:ui no longer uses the templates, they can be removed.

I bumped the version of the snippets package to pick up a change that allows removal of the --template argument.

This extracts the sample code out from the API doc comments, and places them in separate files on disk, allowing running of the examples locally, testing them, and building of slightly larger examples.

This reverts commit 4a720313.

This reverts commit 99daddc2.

Adds MediaQuery.maybeOf to replace calling MediaQuery.of(context, nullOk: true), and removes the nullOk parameter. Also changes MediaQuery.of to return a non-nullable value, and removes many instances of the ! operator, reducing the possible places where a null dereference could occur.