package:build v3.1.0

• Deprecate runBuilder. It has been moved to package:build_runner_core and
  will be removed from package:build. Please note: the currently supported
  ways to run builders are using build_runner on the command line or
  build_test in tests. If you need ongoing support for a different way to run
  builders please get in touch at
  https://github.com/dart-lang/build/discussions.
• Deprecate runPostProcessBuilder. Like runBuilder, it has been moved to
  package:build_runner_core and will be removed from package:build.
• Deprecate AssetWriterSpy. It will be removed.
• Deprecate MultiplexingBuilder. It will be removed.
• Use build_runner_core 9.3.1.