v6.1.0 missing from CHANGELOG

[stkent]

See https://github.com/actions/checkout/blob/f548e57e544e1ff5a4c46bf1e1b8685f8e4a348a/CHANGELOG.md

[pieterocp]

Yeah was never added as part of the backport. Feel free to write it based on v6.0.3...v6.1.0