Move to a Ubuntu version that was acceptable

... when these were last changed

Author: paulo-ferraz-oliveira

.github/workflows/main.yml

@@ -16,7 +16,7 @@ jobs:
     strategy:
       matrix:
         otp_version: ['24', '23', '22', '21']
-        os: [ubuntu-latest]
+        os: [ubuntu-20.04]
 
     steps:
     - uses: actions/checkout@v2