remove unmaintained old Swift image variants

Author: mattpolzin

.github/workflows/tests.yml

@@ -13,29 +13,19 @@ jobs:
       fail-fast: false
       matrix:
         image:
-          - swift:5.2-xenial
-          - swift:5.2-bionic
           - swift:5.2-focal
           - swift:5.2-centos8
-          - swift:5.2-amazonlinux2
-          - swift:5.3-xenial
-          - swift:5.3-bionic
           - swift:5.3-focal
           - swift:5.3-centos8
-          - swift:5.3-amazonlinux2
-          - swift:5.4-xenial
-          - swift:5.4-bionic
           - swift:5.4-focal
           - swift:5.4-centos8
-          - swift:5.4-amazonlinux2
-          - swift:5.5-xenial
-          - swift:5.5-bionic
           - swift:5.5-focal
           - swift:5.5-centos8
-          - swift:5.5-amazonlinux2
-          - swift:5.6-bionic
           - swift:5.6-focal
-          - swift:5.6-amazonlinux2
+          - swift:5.7-focal
+          - swift:5.8-focal
+          - swift:5.9-focal
+          - swift:5.10-focal
     container: ${{ matrix.image }}
     steps:
     - name: Checkout code