[ci] remove ubuntu-18.04

Author: samuelsadok

.github/workflows/compile.yaml

@@ -13,7 +13,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        os: [ubuntu-18.04, ubuntu-latest, windows-latest, macOS-latest]
+        os: [ubuntu-latest, windows-latest, macOS-latest]
         board_version: [v3.6-56V]
         debug: [true]