Fix warnings

[fabianfett]

Fix warnings:

1. Use non deprecated NIOThrowingAsyncSequenceProducer factory method
2. Remove UInt8's conformance to ExpressibleByUnicodeScalarLiteral
3. Fix @retroactive conformance issues in test by fully specifying all types

[codecov-commenter]

Codecov Report

Attention: 23 lines in your changes are missing coverage. Please review.

Comparison is base (69ccfdf) 62.28% compared to head (35f5192) 62.27%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #454      +/-   ##
==========================================
- Coverage   62.28%   62.27%   -0.01%     
==========================================
  Files         124      124              
  Lines        9937     9936       -1     
==========================================
- Hits         6189     6188       -1     
  Misses       3748     3748              

Files	Coverage Δ
Sources/PostgresNIO/New/PSQLRowStream.swift	86.37% <100.00%> (+0.04%)	⬆️
...IO/Utilities/SASLAuthentication+SCRAM-SHA256.swift	47.20% <54.90%> (-0.29%)	⬇️

[fabianfett]

Breaking changes:

3 breaking changes detected in PostgresNIO:
  💔 API breakage: extension UInt8 has removed conformance to ExpressibleByUnicodeScalarLiteral
  💔 API breakage: constructor UInt8.init(unicodeScalarLiteral:) has been removed
  💔 API breakage: typealias UInt8.UnicodeScalarLiteralType has been removed

While those are actual breaking changes, it was 100% wrong that this is part of our public API and noone should have ever relied on this! @gwynne are you fine with deciding that this doesn't require a major version?

[gwynne]

@fabianfett The API breakage checker is more trouble than it's worth most of the time, the false positive rate is astronomical.

[fabianfett]

@gwynne this is not a false positive! We are really removing public API here.