Remove Ubuntu 16.04 since it will be removed on September 20, 2021 (a…

…ctions/runner-images#3287).

Update GNU C++ versions with those that are available both on Ubuntu 18.04 and Ubuntu 20.04 on Github CI.

.github/workflows/ubuntu-dep-apt.yml

@@ -13,8 +13,8 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        os: [ubuntu-16.04, ubuntu-18.04, ubuntu-20.04]
-        compiler: [8, 9]
+        os: [ubuntu-18.04, ubuntu-20.04]
+        compiler: [9, 11]
 
     steps:
     # https://github.com/marketplace/actions/cancel-workflow-action
@@ -32,6 +32,9 @@ jobs:
     - name: Print OS information
       run: lsb_release -a
 
+    - name: Print compiler information
+      run: dpkg --list | grep compiler
+
     - name: Install dependencies
       run: sudo apt-get update && sudo apt-get install -y libx11-dev libdc1394-22-dev libv4l-dev liblapack-dev libopenblas-dev libeigen3-dev libopencv-dev