Bump minimum Swift version to 5.7

Motivation:

Now that Swift 5.9 is GM we should update the supported versions and
remove 5.6

Modifications:

* Update `Package.swift`
* Remove `#if swift(>=5.7)` guards
* Delete the 5.6 docker compose file and make a 5.10 one
* Update docs

Result:

Remove support for Swift 5.6, add 5.10

Package.swift

@@ -1,4 +1,4 @@
-// swift-tools-version:5.6
+// swift-tools-version:5.7
 //===----------------------------------------------------------------------===//
 //
 // This source file is part of the SwiftNIO open source project

README.md

@@ -10,15 +10,16 @@ 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 5.6 and newer. The minimum Swift version supported by SwiftNIO SSH releases are detailed below:
+The most recent versions of SwiftNIO SSH support Swift 5.7 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 ..< 0.6.2` | 5.5.2
-`0.6.2 ...`       | 5.6
+`0.6.2 ..< 0.9.0` | 5.6
+`0.9.0 ...`       | 5.7
 
 ## What does SwiftNIO SSH support?
 

Sources/NIOSSH/Docs.docc/index.md

@@ -12,15 +12,16 @@ 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 5.6 and newer. The minimum Swift version supported by SwiftNIO SSH releases are detailed below:
+The most recent versions of SwiftNIO SSH support Swift 5.7 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 ..< 0.6.2` | 5.5.2
-`0.6.2 ...`       | 5.6
+`0.6.2 ..< 0.9.0` | 5.6
+`0.9.0 ...`       | 5.7
 
 ### What does SwiftNIO SSH support?
 

docker/docker-compose.2204.510.yaml

@@ -0,0 +1,24 @@
+version: "3"
+
+services:
+
+  runtime-setup:
+    image: swift-nio-ssh:22.04-5.10
+    build:
+      args:
+        base_image: "swiftlang/swift:nightly-5.10-jammy"
+
+  documentation-check:
+    image: swift-nio-ssh:22.04-5.10
+
+  test:
+    image: swift-nio-ssh:22.04-5.10
+    environment:
+      - MAX_ALLOCS_ALLOWED_client_server_many_small_commands_per_connection=199900
+      - MAX_ALLOCS_ALLOWED_client_server_one_command_per_connection=1060050
+      - MAX_ALLOCS_ALLOWED_client_server_streaming_large_message_in_small_chunks=43050
+      - IMPORT_CHECK_ARG=--explicit-target-dependency-import-check error
+      #- SANITIZER_ARG=--sanitize=thread
+      - WARN_AS_ERROR_ARG=-Xswiftc -warnings-as-errors
+  shell:
+    image: swift-nio-ssh:22.04-5.10

docker/docker-compose.2204.59.yaml

@@ -6,7 +6,8 @@ services:
     image: swift-nio-ssh:22.04-5.9
     build:
       args:
-        base_image: "swiftlang/swift:nightly-5.9-jammy"
+        ubuntu_version: "jammy"
+        swift_version: "5.9"
 
   documentation-check:
     image: swift-nio-ssh:22.04-5.9