ci: run on ubuntu-20.04

The `ruby/ruby-setup` action no longer officially supports 18.04 and trying to
run against 3.2 fails. Let's update to still a somewhat older Ubuntu version to
make sure we're not breaking things too much.

Author: carlosmn

.github/workflows/ci.yml

@@ -38,7 +38,7 @@ jobs:
       fail-fast: false
       matrix:
         ruby: [ '2.6', '2.7', '3.0', '3.1', '3.2' ]
-        os: [ ubuntu-18.04 ]
+        os: [ ubuntu-20.04 ]
     runs-on: ${{ matrix.os }}
 
     name: Ruby ${{ matrix.ruby }} on ${{ matrix.os }}