Bump minimum Swift version to 5.7 (#160)

* 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

* Remove '--enable-test-discovery'

To see if it's causing CI build getting stuck

---------

Co-authored-by: Yim Lee <[email protected]>

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

@@ -13,7 +13,7 @@ services:
     image: swift-service-lifecycle:20.04-5.7
     environment:
       - SKIP_SIGNAL_TEST=true
-      - FORCE_TEST_DISCOVERY=--enable-test-discovery
+      #- FORCE_TEST_DISCOVERY=--enable-test-discovery
 
   shell:
     image: swift-service-lifecycle:20.04-5.7