Clean up and soft-deprecate AsyncKit

[gwynne]

This update starts the official soft-deprecation of AsyncKit. It should not be used in new code (although it will remain included in Vapor and Fluent 4 for compatibility). It is not expected to receive new updates other than security fixes.

Other changes in this update:

• Require the current Vapor minimum Swift version of 5.7
• The ~ operator on EventLoopFuture is now public as was intended (not that anyone noticed it wasn't 🙂)
• The tests run faster
• CI's been updated
• README and docs updated

[codecov-commenter]

Codecov Report

Merging #106 (1f59f18) into main (eab9edf) will increase coverage by 1.10%. The diff coverage is 96.66%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #106 +/- ## ========================================== + Coverage 95.63% 96.74% +1.10% ========================================== Files 19 19 Lines 963 860 -103 ========================================== - Hits 921 832 -89 + Misses 42 28 -14 ``` | [Files](https://app.codecov.io/gh/vapor/async-kit/pull/106?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=vapor) | Coverage Δ | | |---|---|---| | [...ces/AsyncKit/EventLoop/EventLoop+Concurrency.swift](https://app.codecov.io/gh/vapor/async-kit/pull/106?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=vapor#diff-U291cmNlcy9Bc3luY0tpdC9FdmVudExvb3AvRXZlbnRMb29wK0NvbmN1cnJlbmN5LnN3aWZ0) | `0.00% <ø> (ø)` | | | [Sources/AsyncKit/EventLoop/EventLoop+Flatten.swift](https://app.codecov.io/gh/vapor/async-kit/pull/106?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=vapor#diff-U291cmNlcy9Bc3luY0tpdC9FdmVudExvb3AvRXZlbnRMb29wK0ZsYXR0ZW4uc3dpZnQ=) | `100.00% <ø> (ø)` | | | [Sources/AsyncKit/EventLoop/EventLoop+Future.swift](https://app.codecov.io/gh/vapor/async-kit/pull/106?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=vapor#diff-U291cmNlcy9Bc3luY0tpdC9FdmVudExvb3AvRXZlbnRMb29wK0Z1dHVyZS5zd2lmdA==) | `60.00% <ø> (ø)` | | | [...syncKit/EventLoop/EventLoopGroup+Concurrency.swift](https://app.codecov.io/gh/vapor/async-kit/pull/106?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=vapor#diff-U291cmNlcy9Bc3luY0tpdC9FdmVudExvb3AvRXZlbnRMb29wR3JvdXArQ29uY3VycmVuY3kuc3dpZnQ=) | `50.00% <ø> (ø)` | | | [...s/AsyncKit/EventLoop/EventLoopGroup+Throwing.swift](https://app.codecov.io/gh/vapor/async-kit/pull/106?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=vapor#diff-U291cmNlcy9Bc3luY0tpdC9FdmVudExvb3AvRXZlbnRMb29wR3JvdXArVGhyb3dpbmcuc3dpZnQ=) | `100.00% <ø> (ø)` | | | [.../AsyncKit/EventLoopFuture/Collection+Flatten.swift](https://app.codecov.io/gh/vapor/async-kit/pull/106?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=vapor#diff-U291cmNlcy9Bc3luY0tpdC9FdmVudExvb3BGdXR1cmUvQ29sbGVjdGlvbitGbGF0dGVuLnN3aWZ0) | `100.00% <ø> (ø)` | | | [...ventLoopFuture/EventLoopFutureQueue+Sequence.swift](https://app.codecov.io/gh/vapor/async-kit/pull/106?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=vapor#diff-U291cmNlcy9Bc3luY0tpdC9FdmVudExvb3BGdXR1cmUvRXZlbnRMb29wRnV0dXJlUXVldWUrU2VxdWVuY2Uuc3dpZnQ=) | `100.00% <ø> (ø)` | | | [...syncKit/EventLoopFuture/EventLoopFutureQueue.swift](https://app.codecov.io/gh/vapor/async-kit/pull/106?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=vapor#diff-U291cmNlcy9Bc3luY0tpdC9FdmVudExvb3BGdXR1cmUvRXZlbnRMb29wRnV0dXJlUXVldWUuc3dpZnQ=) | `100.00% <ø> (ø)` | | | [...s/AsyncKit/EventLoopFuture/Future+Collection.swift](https://app.codecov.io/gh/vapor/async-kit/pull/106?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=vapor#diff-U291cmNlcy9Bc3luY0tpdC9FdmVudExvb3BGdXR1cmUvRnV0dXJlK0NvbGxlY3Rpb24uc3dpZnQ=) | `100.00% <ø> (ø)` | | | [...AsyncKit/EventLoopFuture/Future+Conjunctions.swift](https://app.codecov.io/gh/vapor/async-kit/pull/106?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=vapor#diff-U291cmNlcy9Bc3luY0tpdC9FdmVudExvb3BGdXR1cmUvRnV0dXJlK0Nvbmp1bmN0aW9ucy5zd2lmdA==) | `99.69% <ø> (ø)` | | | ... and [5 more](https://app.codecov.io/gh/vapor/async-kit/pull/106?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=vapor) | |

[github-advanced-security[bot]]

This pull request sets up GitHub code scanning for this repository. Once the scans have completed and the checks have passed, the analysis results for this pull request branch will appear on this overview. Once you merge this pull request, the 'Security' tab will show more code scanning analysis results (for example, for the default branch). Depending on your configuration and choice of analysis tool, future pull requests will be annotated with code scanning analysis results. For more information about GitHub code scanning, check out the documentation.

[penny-for-vapor[bot]]

These changes are now available in 1.19.0