ci: Drop ubuntu-16.04 (#513)

.github/workflows/test-action.yml

@@ -13,7 +13,6 @@ jobs:
         os:
           - 'ubuntu-20.04'
           - 'ubuntu-18.04'
-          - 'ubuntu-16.04'
           - 'macos-latest'
           - 'windows-latest'
         hugo-version:

.github/workflows/test.yml

@@ -18,7 +18,6 @@ jobs:
         os:
           - 'ubuntu-20.04'
           - 'ubuntu-18.04'
-          - 'ubuntu-16.04'
           - 'macos-latest'
           - 'windows-latest'
     steps:

README.md

@@ -23,7 +23,7 @@ We no longer build or pull a Hugo docker image.
 Thanks to this change, we can complete this action in less than a few seconds.
 (A docker base action was taking about 1 min or more execution time to build and pull a docker image.)
 
-| OS (runs-on) | ubuntu-18.04 | macos-latest | windows-2019 |
+| OS (runs-on) | ubuntu-18.04, ubuntu-20.04 | macos-latest | windows-2019 |
 |---|:---:|:---:|:---:|
 | Support | ✅️ | ✅️ | ✅️ |