67939 Remove unused Gold methods (cla: yes, framework, team, waiting for tree to go green)

67940 Fix NestedScrollView sample code (a: quality, cla: yes, d: api docs, d: examples, documentation, framework, waiting for tree to go green)

67941 Migrate yet even more widget framework tests to null safety. (a: accessibility, cla: yes, framework)

67946 Migrate Scaffold SnackBars in the framework to ScaffoldMessenger (a: tests, cla: yes, d: api docs, d: examples, documentation, f: material design, framework, team, waiting for tree to go green)

67947 Deprecate old SnackBar methods (cla: yes, f: material design, framework, severe: API break, severe: new feature, waiting for tree to go green)