.github/workflows/codespell.yml: remove ubuntu-18.04

Author: RandyMcMillan

.github/workflows/codespell.yml

@@ -11,7 +11,7 @@ jobs:
     runs-on: ${{ matrix.os }}
     strategy:
       matrix:
-        os: [ubuntu-18.04, ubuntu-20.04, ubuntu-latest]
+        os: [ubuntu-20.04, ubuntu-latest]
     steps:
       - uses: styfle/[email protected]
         with: