Raise minimum Swift version to 5.5

[fabianfett]

Motivation:

Now that 5.7 has been released NIO is due to drop support for 5.4. PostgresNIO Swift must do the same.

Modifications:

• Update Package.swift to require Swift 5.5
• Update the README.md
• Move one integration test case to AsyncTests, where it fits better

Result:

Minimum supported Swift version is 5.5