Back deploy concurrency to older Apple Platforms

[sevki]

Enables concurrency features on macOS 10.15 et al. Drops support for Swift 5.5.0 and 5.5.1 to match SwiftNIO

[codecov-commenter]

Codecov Report

Merging #97 (edb6c1c) into main (3be4b64) will not change coverage. The diff coverage is n/a.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #97 +/- ## ======================================= Coverage 95.82% 95.82% ======================================= Files 19 19 Lines 1199 1199 ======================================= Hits 1149 1149 Misses 50 50 ``` | Flag | Coverage Δ | | |---|---|---| | unittests | `95.82% <ø> (ø)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=vapor#carryforward-flags-in-the-pull-request-comment) to find out more. | [Impacted Files](https://codecov.io/gh/vapor/async-kit/pull/97?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://codecov.io/gh/vapor/async-kit/pull/97/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=vapor#diff-U291cmNlcy9Bc3luY0tpdC9FdmVudExvb3AvRXZlbnRMb29wK0NvbmN1cnJlbmN5LnN3aWZ0) | `100.00% <ø> (ø)` | | | [...syncKit/EventLoop/EventLoopGroup+Concurrency.swift](https://codecov.io/gh/vapor/async-kit/pull/97/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=vapor#diff-U291cmNlcy9Bc3luY0tpdC9FdmVudExvb3AvRXZlbnRMb29wR3JvdXArQ29uY3VycmVuY3kuc3dpZnQ=) | `100.00% <ø> (ø)` | |

[VaporBot]

These changes are now available in 1.15.0