Move to Ubuntu 20.04; 18.04 is deprecated

Author: sedwards2009

.github/workflows/linux.yml

@@ -3,14 +3,14 @@ on:
     types: [created]
 jobs:
   build:
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-20.04
     strategy:
       matrix: 
         arch: ['x64']
     steps:
       - uses: actions/setup-node@master
         with:
-          node-version: "16.x"
+          node-version: "18.x"
       - name: Set up Python 3.8
         uses: actions/setup-python@v1
         with: