Drop Swift 5.5 support. (#140)

Motivation:

Per our support policy, with the release of 5.8 we are no longer support
Swift 5.5.

Modifications:

Drop infrastructure supporting 5.5
Remove conditional compilation guards.

Result:

Nice clean Swift 5.6+ repo.

README.md

@@ -10,14 +10,15 @@ There are a number of reasons to provide a programmatic SSH implementation. One
 
 Another good reason to provide programmatic SSH is that it is not uncommon for services to need to interact with other services in a way that involves running commands. While `Process` solves this for the local use-case, sometimes the commands that need to be invoked are remote. While `Process` could launch an `ssh` client as a sub-process in order to run this invocation, it can be substantially more straightforward to simply invoke SSH directly. This is [`libssh2`](https://www.libssh2.org)'s target use-case. SwiftNIO SSH provides the equivalent of the networking and cryptographic layer of libssh2, allowing motivated users to drive SSH sessions directly from within Swift services.
 
-The most recent versions of SwiftNIO SSH support Swift 5.5.2 and newer. The minimum Swift version supported by SwiftNIO SSH releases are detailed below:
+The most recent versions of SwiftNIO SSH support Swift 5.6 and newer. The minimum Swift version supported by SwiftNIO SSH releases are detailed below:
 
 SwiftNIO SSH      | Minimum Swift Version
 ------------------|----------------------
 `0.0.0 ..< 0.3.0` | 5.1
 `0.3.0 ..< 0.4.0` | 5.2
 `0.4.0 ..< 0.5.0` | 5.4
-`0.5.0 ...`       | 5.5.2
+`0.5.0 ..< 0.6.2` | 5.5.2
+`0.6.2 ...`       | 5.6
 
 ## What does SwiftNIO SSH support?
 

Sources/NIOSSH/Child Channels/SSHChannelData.swift

@@ -34,10 +34,8 @@ public struct SSHChannelData {
 
 extension SSHChannelData: Equatable {}
 
-#if swift(>=5.6)
 @available(*, unavailable)
 extension SSHChannelData: Sendable {}
-#endif
 
 extension SSHChannelData {
     /// The type of this channel data. Regular ``SSHChannelData/DataType/channel`` data is the standard type of data on an `SSHChannel`,

Sources/NIOSSH/Docs.docc/index.md

@@ -12,14 +12,15 @@ There are a number of reasons to provide a programmatic SSH implementation. One
 
 Another good reason to provide programmatic SSH is that it is not uncommon for services to need to interact with other services in a way that involves running commands. While `Process` solves this for the local use-case, sometimes the commands that need to be invoked are remote. While `Process` could launch an `ssh` client as a sub-process in order to run this invocation, it can be substantially more straightforward to simply invoke SSH directly. This is [`libssh2`](https://www.libssh2.org)'s target use-case. SwiftNIO SSH provides the equivalent of the networking and cryptographic layer of libssh2, allowing motivated users to drive SSH sessions directly from within Swift services.
 
-The most recent versions of SwiftNIO SSH support Swift 5.5.2 and newer. The minimum Swift version supported by SwiftNIO SSH releases are detailed below:
+The most recent versions of SwiftNIO SSH support Swift 5.6 and newer. The minimum Swift version supported by SwiftNIO SSH releases are detailed below:
 
 SwiftNIO SSH      | Minimum Swift Version
 ------------------|----------------------
 `0.0.0 ..< 0.3.0` | 5.1
 `0.3.0 ..< 0.4.0` | 5.2
 `0.4.0 ..< 0.5.0` | 5.4
-`0.5.0 ...`       | 5.5.2
+`0.5.0 ..< 0.6.2` | 5.5.2
+`0.6.2 ...`       | 5.6
 
 ### What does SwiftNIO SSH support?