Use Ubuntu 20.04 and remove Ubuntu 18.04 from workflows

- `ubuntu-latest` now resolves to `ubuntu-22.04`. Explicit pinning
  is needed.

- cherry-pick #14238

Signed-off-by: szubersk <szuberskidamian@gmail.com>

.github/workflows/zfs-tests-functional.yml

@@ -9,7 +9,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        os: [18.04, 20.04]
+        os: [20.04]
     runs-on: ubuntu-${{ matrix.os }}
     steps:
     - uses: actions/checkout@v3