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`
* Delete the <5.7 docker compose files and make a 5.10 one
* Update the 5.9 docker file to stop using nightlies

Result:

Remove support for Swift 5.6, add 5.10

Author: rnro

Package.swift

@@ -1,4 +1,4 @@
-// swift-tools-version:5.6
+// swift-tools-version:5.7
 
 import PackageDescription
 

docker/Dockerfile

@@ -1,4 +1,4 @@
-ARG swift_version=5.6
+ARG swift_version=5.7
 ARG ubuntu_version=focal
 ARG base_image=swift:$swift_version-$ubuntu_version
 FROM $base_image

docker/docker-compose.1604.51.yaml

@@ -0,0 +1,18 @@
+version: "3"
+
+services:
+
+  runtime-setup:
+    image: swift-service-lifecycle:22.04-5.10
+    build:
+      args:
+        base_image: "swiftlang/swift:nightly-5.10-jammy"
+
+  test:
+    image: swift-service-lifecycle:22.04-5.10
+    environment:
+      - SKIP_SIGNAL_TEST=true
+      - FORCE_TEST_DISCOVERY=--enable-test-discovery
+
+  shell:
+    image: swift-service-lifecycle:22.04-5.10

docker/docker-compose.1804.50.yaml

@@ -6,7 +6,8 @@ services:
     image: swift-service-lifecycle:22.04-5.9
     build:
       args:
-        base_image: "swiftlang/swift:nightly-5.9-jammy"
+        ubuntu_version: "jammy"
+        swift_version: "5.9"
 
   test:
     image: swift-service-lifecycle:22.04-5.9