24601 [macos] Reland default metal and also check if the system supports metal before defaulting to it (cla: yes)

24602 Skip flaky webkit shadermask test (cla: yes)

24612 Allow calls to legacyMakeTypeface in the AssetFontManager (cla: yes, waiting for tree to go green)

24616 Add third_party/libpng repository and roll buildroot (cla: yes)

24623 [linux] Fix text selection via Shift+Home/End (cla: yes)

24635 [flutter_releases] Flutter Engine Cherrypicks (cla: yes, platform-android)

24640 Enables semantics when voice control is turned on (cla: yes, platform-ios, waiting for tree to go green)