72163 Add some new examples to Actions and Shortcuts (cla: yes, f: material design, framework)

72297 Migrate some material doc samples to null safety. (cla: yes, f: material design, framework, waiting for tree to go green)

72300 Fixed issue for SliverAppBar collapsedHeight (cla: yes, f: material design, f: scrolling, framework, waiting for tree to go green)

72303 Migrate some more material doc samples to null safety. (cla: yes, f: material design, framework, waiting for tree to go green)

72308 Add ScrollbarTheme/ScrollbarThemeData (cla: yes, f: material design, f: scrolling, framework, severe: new feature, waiting for tree to go green)