GH Actions: Drop old Swift versions

... ain't supported by NIO anymore.

Author: helje5

.github/workflows/swift.yml

@@ -1,4 +1,4 @@
-name: Build and Test
+name: Build
 
 on:
   push:
@@ -13,10 +13,8 @@ jobs:
       fail-fast: false
       matrix:
         image:
-          - swift:5.2.5-xenial
-          - swift:5.3.2-xenial
-          - swift:5.3.2-bionic
           - swift:5.5.0-focal
+          - swift:5.6.1-bionic
     container: ${{ matrix.image }}
     steps:
       - name: Checkout Repository