GHActions: Drop Swift 5.0.3

Doesn't work w/ --enable-test-discovery.

Author: helje5

.github/workflows/swift.yml

@@ -13,7 +13,6 @@ jobs:
       fail-fast: false
       matrix:
         image:
-          - swift:5.0.3-xenial
           - swift:5.1.5-xenial
           - swift:5.3.2-bionic
           - swift:5.9.2-focal