Reduce minimum os requirement and increase swift compiler version

[patskovn]

With Xcode 13.2 we can use swift concurrency on older OSes. I followed swift-nio's implementation of this and adopted it in grpc-swift.
Tested with Xcode 13.0 and Xcode 13.2. Also tested in my project that use both Server and Client implementations. Everything compiles and runs.

[linux-foundation-easycla]

• ❌ The commit (9413638) is missing the User's ID, preventing the EasyCLA check. Consult GitHub Help to resolve.For further assistance with EasyCLA, please submit a support request ticket.

[glbrntt]

Thanks for the PR @Nekitosss -- however, you'll need to sign the CLA to contribute to the project.

[patskovn]

Yeah, I know, sorry. Need to make this official with my employer :) Trying to do this asap

[patskovn]

Update:
My change was approved inside the company. Trying to resolve official bureaucracy stuff and find someone who can be my CLA manager. Hope this will be done soon

[patskovn]

Ok, I cannot sign this CLA. After succes s signing in DocuSign, I redirected into my previous pull request to this repo #1272
and this commit remains unsigned. Sorry