Update CI to Ubuntu 20.04

.github/workflows/ci.yaml

@@ -20,5 +20,5 @@ jobs:
     - uses: actions/checkout@v2
 
     - name: Build and TesT
-      run: npx --package mini-cross@0.15.2 mc --no-tty ubuntu:16.04 .mc/rebuild.sh
+      run: npx --package mini-cross@0.15.2 mc --no-tty ubuntu:20.04 .mc/rebuild.sh
 

.mc/ubuntu:16.04.yaml → .mc/ubuntu:20.04.yaml

@@ -1,5 +1,5 @@
 ---
-base: ubuntu:16.04
+base: ubuntu:20.04
 install:
  - cmake
  - g++