Update ubuntu image for CI

Author: iabudiab

.github/workflows/ci.yml

@@ -26,8 +26,8 @@ jobs:
       - name: iOS - ${{ matrix.destination }}
         run: set -o pipefail && xcodebuild -scheme SwiftkubeModel -destination "${{ matrix.destination }}" clean test | xcpretty
   linux:
-    runs-on: ubuntu-18.04
-    container: swift:5.5
+    runs-on: ubuntu-latest
+    container: swift:5.6
     steps:
     - uses: actions/checkout@v2
     - name: Build