update linux swift images used for later versions of swift.

Author: mattpolzin

.github/workflows/tests.yml

@@ -33,10 +33,9 @@ jobs:
           - swift:5.5-focal
           - swift:5.5-centos8
           - swift:5.5-amazonlinux2
-          - swift:5.6-xenial
           - swift:5.6-bionic
           - swift:5.6-focal
-          - swift:5.6-centos8
+          - swift:5.6-centos7
           - swift:5.6-amazonlinux2
     container: ${{ matrix.image }}
     steps: