v4.0.0-rc3

This is the third release candidate for the first official release of Lean 4.

It replaces v4.0.0-rc2, which was not properly prepared as a release, so #eval Lean.toolchain and lean --version gave incorrect answers. Additionally, there are changes to the CI for building releases and additions to RELEASES.md for previously undocumented breaking changes.

What's Changed

• chore: remove binaries before building them by @tydeu in #2450
• doc: improve doc-string for Meta.getConst? by @semorrison in #2454
• fix: make sure refine preserves pre-existing natural mvars by @thorimur in #2435
• doc: fix typos by @marcusrossel in #2467
• fix: nix: If src is a dir, assume the lean file has the full path by @nomeata in #2465
• feat: add labels from comments by @semorrison in #2460
• feat: create release at lean4-pr-releases for each PR by @semorrison in #2448
• chore: update CI to create official releases by @semorrison in #2472
• test: basic tests exercising rw by @semorrison in #2397
• chore: use bash-compatible SemVer regex by @semorrison in #2475
• chore: add introduction to RELEASES.md by @semorrison in #2476
• chore: update README to reflect beginning stable releases by @semorrison in #2477
• chore: remove - from semver prerelease by @semorrison in #2481
• doc: update RELEASES.md for change to getConst? by @semorrison in #2482

Full Changelog: v4.0.0-rc2...v4.0.0-rc3