Fix NIO "Lock" deprecation warnings

[gwynne]

Also drops support for Swift 5.4, since NIO has already done so.

More than half of the changes included in this update were contributed by @Colgates in #95 - thanks again!

[codecov-commenter]

Codecov Report

Merging #96 (9b3d3e7) into main (c3329e4) will not change coverage. The diff coverage is 100.00%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #96 +/- ## ======================================= Coverage 95.82% 95.82% ======================================= Files 19 19 Lines 1199 1199 ======================================= Hits 1149 1149 Misses 50 50 ``` | Flag | Coverage Δ | | |---|---|---| | unittests | `95.82% <100.00%> (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/96?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=vapor) | Coverage Δ | | |---|---|---| | [.../ConnectionPool/EventLoopGroupConnectionPool.swift](https://codecov.io/gh/vapor/async-kit/pull/96/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=vapor#diff-U291cmNlcy9Bc3luY0tpdC9Db25uZWN0aW9uUG9vbC9FdmVudExvb3BHcm91cENvbm5lY3Rpb25Qb29sLnN3aWZ0) | `94.77% <100.00%> (ø)` | |

[VaporBot]

These changes are now available in 1.14.0