Update to modern runner

Author: jpedley

.github/workflows/main.yml

@@ -5,7 +5,7 @@ on: push
 jobs:
   publish:
     name: Publish library to PlatformIO registry
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-22.04
 
     steps:
     - uses: actions/checkout@v3