3575 [path_provider] Bump platform packages to NNBD stable (cla: yes, nnbd, p: path_provider, platform-linux, platform-macos, platform-windows)

3584 [url_launcher] Update platforms to NNBD stable (cla: yes, nnbd, p: url_launcher, platform-linux, platform-macos, platform-web, platform-windows)

3595 [shared_preferences] Bump platform versions to NNBD stable (cla: yes, nnbd, p: shared_preferences, platform-linux, platform-macos, platform-web, platform-windows)

3644 [path_provider] Migrate all integration tests to NNBD (cla: yes, p: path_provider, platform-linux, platform-macos, platform-windows)