Bump minimum supported Swift version to 5.5
Motivation
RediStack 1.x is going to be supported at least until 2.x is released, which is a few months away.
Developers who are on later versions of Swift will adopt later versions of NIO and other libraries, and after we merged !183 (merged) we required NIO 2.42.x which has a minimum Swift version of 5.5
Changes
This back ports the CI config from master
into this branch, with updates to the README to reflect the changes