Upgrade Ubuntu version from `18.04` to `20.04` (#11395)

Co-authored-by: Jirka Borovec <Borda@users.noreply.github.com>
Co-authored-by: Carlos Mocholi <carlossmocholi@gmail.com>

.github/workflows/ci_test-full.yml

@@ -17,13 +17,13 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        os: [ubuntu-18.04, windows-2019, macOS-10.15]
+        os: [ubuntu-20.04, windows-2019, macOS-10.15]
         python-version: ["3.7", "3.9"]  # minimum, maximum
         requires: ["oldest", "latest"]
         release: ["stable"]
         exclude:
           # Skip if torch<1.8 and py3.9 on Linux: https://github.com/pytorch/pytorch/issues/50014
-          - {os: ubuntu-18.04, python-version: "3.9", requires: "oldest", release: "stable"}
+          - {os: ubuntu-20.04, python-version: "3.9", requires: "oldest"}
         #include:
           # nightly: add when there's a release candidate
           #- {os: ubuntu-20.04, python-version: "3.10", requires: "latest", release: "pre"}