Release preparations for 1.1

[lorentey]

This PR is collecting final preparations for the 1.1.0 release.

Changes so far:

• Bump the required Swift toolchain version to 5.7
• Adjust BitArray's description
• Upgrade to modern generic syntax (some Sequence<Int>) where appropriate
• Update & regenerate DocC catalogs
• Bump copyright dates
• Small build config fixes

(More changes will be added as I go reviewing the current state.)

Checklist

• [X] I've read the Contribution Guidelines
• [X] My contributions are licensed under the Swift license.
• [X] I've followed the coding style of the rest of the project.
• [ ] I've added tests covering all new code paths my change adds to the project (if appropriate).
• [ ] I've added benchmarks covering new functionality (if appropriate).
• [X] I've verified that my change does not break any existing tests or introduce unexplained benchmark regressions.
• [X] I've updated the documentation if necessary.

[lorentey]

@swift-ci test