release_2.21.0.dev4

New Features

• use default upstream and upgrade shellcheck (#20702)

• Add test retries to remaining backends (#20606)

Bug Fixes

• docker buildx output format changed in 0.13.1 (#20693)

• Core: Fix __defaults__ to also apply for generated file targets. (#20649)

Documentation

• Update welcome-to-pants.mdx - fix link to sample users (#20688)

• Link to rate limit issue in GitHub Actions remote cache provider docs (#20690)

• docs: update docs for debian_package target (#20665)

• update docs to use proper docker sha256 format (#20685)