Document that the backtracking resolver is the current default

[jeffwidman]

I think updating these docs was accidentally overlooked when the https://github.com/jazzband/pip-tools/releases/tag/7.0.0 release happened.

Contributor checklist

• Provided the tests for the changes.
• Assure PR title is short, clear, and good to be included in the user-oriented changelog

Maintainer checklist

• Assure one of these labels is present: backwards incompatible, feature, enhancement, deprecation, bug, dependency, docs or skip-changelog as they determine changelog listing.
• Assign the PR to an existing or new milestone for the target version (following Semantic Versioning).