Github Actions: skip building for Ubuntu 16.04

First, 16.04 only supports libssl 1.0 (officially) that links fine but will not find the library at runtime.
Second, 16.04 is old (LTS is still valid tho).

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 ]
         build: [ cli, cli-mt, cli-ansi, cli-ansi-mt, gui, gui-mt ]
         include:
           - build: cli

.github/workflows/package.yml

@@ -9,7 +9,6 @@ jobs:
   ubuntu:
     strategy:
       matrix:
-        os: [ ubuntu-16.04, ubuntu-18.04 ]
         component: [ cli, gui ]
         include:
           - component: cli
@@ -17,7 +16,7 @@ jobs:
           - component: gui
             build-opts: --no-cli --gui -t Release --pkg-type deb
     name: ${{matrix.os}} (${{matrix.component}})
-    runs-on: ${{matrix.os}}
+    runs-on: ubuntu-18.04
     steps:
       - uses: actions/checkout@v1