Github Actions: skip building/package for Ubuntu 16.04

16.04 only supports libssl 1.0 (officially).
If requested, we could consider supporting it but no point going through the hassle if it has no users.

.github/workflows/CI.yml

@@ -11,7 +11,7 @@ jobs:
   compile:
     strategy:
       matrix:
-        os: [ ubuntu-16.04, ubuntu-18.04, macos-latest, windows-latest ]
+        os: [ ubuntu-18.04, macos-latest, windows-latest ]
 
     name: ${{matrix.os}}
     runs-on: ${{matrix.os}}

.github/workflows/package.yml

@@ -7,11 +7,8 @@ on:
 jobs:
   # Debian package for ubuntu.
   ubuntu:
-    strategy:
-      matrix:
-        os: [ ubuntu-16.04, ubuntu-18.04 ]
-    name: ${{matrix.os}}
-    runs-on: ${{matrix.os}}
+    name: Ubuntu 18.04
+    runs-on: ubuntu-18.04
     steps:
       - uses: actions/checkout@v1