Merge pull request #19 from arduino/remove-ubuntu-18.04

Removed ubuntu-18.04 runner from CI

Author: umbynos

.github/workflows/test.yaml

@@ -11,7 +11,6 @@ jobs:
         operating-system:
           [
             ubuntu-latest,
-            ubuntu-18.04,
             windows-latest,
             macos-latest,
           ]