3.38 beta branch info

Tasks

• Determine a SHA cutoff point.
• Verify the SHA cutoff includes the Dart 3.10.0-290.1.beta beta, and has rolled successfully into google3.
• Create a release branch.
• Create a recipes branch.
• Stamp the branch (flutter-3.38-candidate.0) into bin/internal/release-candidate-branch.version and set the version in bin/internal/engine.version to the SHA cutoff point.
• Update engine.version to the previous commit where release-candidate-branch.version and engine.version were created.
• Merge any cherry picks pre-release should they come up and update engine.version if needed.
• Build and run post-submits
• Publish 3.38.0-0.0.pre

cc @iinozemtsev @justinmc