Update Ubuntu image in CI

Author: valeros

.github/workflows/examples.yml

@@ -7,7 +7,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        os: [ubuntu-16.04, windows-latest, macos-latest]
+        os: [ubuntu-18.04, windows-latest, macos-latest]
         python-version: [3.7]
         example:
           - "examples/mbed-rtos-blink-baremetal"